Can I use a one-way ticket to come to the UK? Or do I need to show a return ticket to be able to get in?

You don't need a return ticket, but with you having a boyfriend there, I'd strongly recommend you to get one. If nothing else, a random fully-refundable ticket for a date around the time you suspect you'll return.

Also, if staying longer than a month, bring every single piece of documentation proving your ties to the US (signed/stamped employment/school letter with contact details, utility bills, proof of family ties etc.). Essentially anything and everything answering the question "why would she go back home, and not just remain illegally?"

In addition, you'll need to establish you won't resort to employment or public funds, so proof of income should be brought along.

All of this said, you can now use an e-passport gate at the largest airports, which involves talking to no one. And yes, you are indeed exempt from needing an entry stamp.

A return or round trip ticket is not required.

However, depending on your entry experience, it may be helpful to demonstrate your intention to return or at least leave. If you are routed to a Border Force Officer, they must be satisfied you are not an overstay risk.

If cash is an issue, you can often find award tickets for less than most credit card sign up bonuses. There may be a redeposit fee but it's nominal.

I've entered the UK several times on one-way tickets with no issue, but my travel patterns are not indicative of an overstay risk. Last time I used the new ePassport gates which took about 20 seconds.

It's also worth looking into a "Miscellaneous_charges_order" (MCO). Its like an airline ticket, issued by an airline in your home country, but shows a currency amount rather than a flight booking. It is convertable into an airline ticket on demand, with any airline (almost). It is refundable in your home country if not used, but not refundable for cash anywhere else in the world. These proved useful in South American countries to show I had means of getting home if I needed to. The point about all those rules attached to the MCO is that they are designed by the airlines to reassure border agents that you will not just cash in the ticket and overstay.

If you don't use it to get home, all you lose is the potential interest on the money that you "lent" the airline

(This experience is from some time ago, do check if they are still available).

All the answers above are good, but I have a bit of additional advice.

I'm British, and my wife is American and works for a US airline, so we frequently have family coming to the UK using her cheap travel benefits, who usually stay for a while, so I have some experience on what to expect.

If you don't have a return ticket, your first problem will not be at the UK border, but when you try and board the flight to the UK in the US. The airline will not let you board unless you can show a return or onward flight from the UK, or a valid residency permit.

However, if you have a one-way ticket, you can buy a cheap onward flight/train to somewhere in Europe (EasyJet/RyanAir/EuroStar). The airline doesn't care where and won't check whether it's a return ticket. They just don't want to be fined by the UK government if you are denied entry on arrival and don't have a way out.

On arrival, if you have a new-style US passport with a chip you can use the eGates. You just scan and walk through. It's unlikely you'll speak to anyone if you fly in to London Heathrow, other UK airports are still working it out though. Saying that, it's not actually automatic, someone is looking at a screen and can redirect you to an immigration officer. Sadly like the US, your name's origin, age, ethnicity, travel history and place of birth (if not the US) will have a bearing on whether you are profiled, as will your flight's return date.

If you are profiled then rule no.1 is tell the truth. If they catch you out in a lie, then being refused entry is a whole world of pain, and will cause you ongoing problems travelling to all countries in the future. However that doesn't mean you can't be "economical" with the truth. Why not buy a flexible return, but set the initial return date for 2 weeks? If things are good, extend the ticket out. Once you have passed immigration, as long as you leave before your visitor visa expires in 6 months, then that's OK. And telling the officer that you are just here for a 2-week visit seeing friends, will get you through a lot quicker than getting in to the details of your relationship.

Also (unless the law has changed), if you arrived as a visitor you will have to leave the UK and apply for your UK visa from the US, if that's where you currently live.

Finally don't forget about health insurance. Annual travel policies usually only allow you to stay in one location for a maximum of 6 weeks. If you don't have proof of UK residency and have to use the UK's NHS, you will get charged. Although the bills are nowhere as eye-watering as in the US, they can be substantial. One option if you have US health insurance, you maybe able to get it extended to have worldwide cover.

On an aside, I did something similar a very long time ago. Flew to Asia with a return ticket to see my girlfriend and ended up staying there [legally :)] for 15 years. Whatever happens, living abroad is a wonderful experience and even if the relationship doesn't work out, you'll discover a lot more about yourself and make life-long friends. Good luck.

The UK gives advice to applicants for visas (who are from countries which do not have a visa waiver, like Nigeria). The UK says specifically Don't bother buying an airline ticket; they will not convince us of your intent to leave the UK at the end of your planned visit.

Immigration is there to enforce the immigration laws: mainly to make sure visitors don't overstay, seek employ, rely on the dole (public services) or do crime/terror. They presume immigrant intent (the bad thing) and then allow you to show the probabilities that this is not so. For many e.g. American visitors, this is so automated that it's usually a card-swipe at a kiosk.

You may skate by with the automated system. However if not, your situation raises a bunch of questions, and you need to be able to answer them smartly or you'll get a refusal.

• Since your goal is a permanent relationship with a Briton, what keeps you from overstaying? Here you need to show lots of home ties that compel your return to the US: job, responsibilities like civic volunteerism, assets like a house and car, etc.
• How can an open return date work for you? Don't you have a job/life/family to get back to? That'll require some explaining, because it casts a shadow on the first point, like how can that possibly work with your employer? Immigration does not like wanderers because they are a high risk for overstaying and flaunting immigration law.

• How does money work for this visit? Will you get in trouble with money and rely on UK social services (the dole)? Noting that US adults usually get their healthcare from their employer, how does your health insurance work? They will want to see a realistic attitude and documentation not only of means appropriate to this trip, but also that this isn't your whole life savings.

  • It is simply not credible that someone would burn their entire life savings on a tourist visit to the UK -- for visitors from some countries, it's a redflag that they seek illegal employ or are up to no good.

• Back to money, again how can that work? Will you take up employment in the UK to make ends meet?

You need to be ready to answer those questions. You seem more prepared than most I see here, and that can play to your advantage - e.g. if you say "Well we certainly won't marry; if I get a proposal, I'll apply for a Marriage Visitor Visa, and we'll live in the US." (and you better be able to answer how your partner will qualify to enter the US for that purpose).

You have a lot of answers related to your current plan of staying up to six months. This answer questions that whole plan.

Your goal is to work out where your current long distance relationship is going, and if it goes well to be able to marry your boyfriend and move to the UK. For that to be feasible, you need to keep a good record with the UK immigration authorities over the long term. You may also need your savings to help satisfy the financial requirements for a settlement visa as a spouse.

The six month visit plan is risky for several reasons. It is hard to argue you have ties and ongoing activities in the US that ensure your return if you can drop everything for six months. That is a long time to put a job on hold, or delay education, or keep up somewhere to live without actually living there. In addition to the immigration and travel issues, the more you shut your life down now, the worse you will feel if the current relationship does not work out.

As an alternative, maintain your life and ties in the US. Get/keep a job, make education commitments, have a home, even if it is just a room in a shared apartment. Plan a relatively short visit to the UK, two or three weeks, with a definite return date. The return ticket is not important. The plan is. "I am going to return home on [date], which will allow a week before I start classes at [college]" is the sort of attitude that reassures immigration officers. If the first visit goes well, visit again in the future. Between your trips to the UK, ask your boyfriend to make similar short visits to you in the US. In the process, you will be establishing a travel record as someone who makes plans and follows them.

A US citizen with an established life in the US, or a UK citizen with an established life in the UK, spending two or three weeks in the other country is routine, with very little risk of being denied entry.