Accommodation options in London for three months in summer

It's difficult... £143 / week (~£625) a month is a typical price for a long term stay in a London very small studio flat or a decent room in a shared apartment. These often have minimum contracts - 12 months minimum is quite common.

In Zone 1 or 2, finding decent accommodation for £143 a week isn't easy even if you were staying a year. If you can't find a regular apartment that allows 3 month contracts, your best options are:

• The option most likely to be in your price range is to try to find a houseshare where there's an empty room people are looking to fill for a few months. For example, three or four locals sharing an apartment where one person moved out a few months before the contract expired. http://spareroom.co.uk and http://gumtree.co.uk are the best places for this in my experience, and Spare Room's advanced search has an option where you can set maximum duration as 3 months. But be prepared for it to take a week or two to find somewhere suitable - they'll probably want to meet you first to see if you're someone they're happy to live with for three months. Expect to need a week or two in a hostel while you visit flats and contact people.
• If you don't mind moving around a lot, you could try http://couchsurfing.com where friendly people let people stay for free as a way to meet interesting people from around the world. But you'll need to move on every 3-4 day, there's HUGE demand in London so most people won't even get back to you, and you'll need to work hard to find people who'll let you stay, especially if you've never used the site before and don't have people saying you're a good guest. They'll be a long way from zone 1 or 2.
• Accommodation sites like https://www.airbnb.co.uk/ where locals rent out rooms to visitors are popular - but these aren't cheap (cheaper than hotels, but more expensive than renting), and it'd be unlikely to be in zone 1 or 2. They're usually aimed at tourists staying a few weeks: you might be able to negotiate a discount since you'd save the owner the hassle of finding new people for 3 months, but this might be tricky since summer is the busy season

• You could look for accommodation sites with a "short let" category. For example popular ad listing site Gumtree has such a category: http://www.gumtree.com/short-term-rent/london/short+let - but these are aimed at tourists and you'd be lucky to find something cheaper than £300 / week.

• You could also negotiate with regular landlords:

  • Some landlords allow people to leave before their contract is up if that person takes responsibility for finding their replacement. You might be able to rent a regular 12-month appartment then find someone to take over from you for the last 9 months. This is risky, but demand for accommodation in London is always high (put an ad on Gumtree etc and people will contact you). If you're flexible with dates (you might end up staying a week or two longer than planned while you find your replacement), this is an option.
  • From the point of view of a landlord looking to let out low-cost accommodation, late August or September is one of the best times of year to advertise for low-end 1-year lets because London fills up with international students. Landlords sometimes put rent up slightly because demand outstrips supply even more than usual. You might be able to persuade someone advertising for regular long-term accommodation in early summer to consider making an exception and allowing a 3-month contract, because they'll then be advertising for your replacement at the best time of year.

Another option would be to rent a room in a university accommodation. Rooms that are usually occupied by Bachelor students become vacant at the end of May/beginning of June, when the second term ends and the summer holidays begin. At which point universities try to rent these out to summer schools, language course students, and all those people who need a room for the summer in London.

For example, Imperial College London rents halls out for the summer. Some of the halls are in Zones 1 and 2. Moreover, the price range is likely fit in your budget. It will be up to you to look for options.