Documenting money taking out of UK

At today's exchange rate, that's slightly less than €10,000, but who knows what the exchange rate will be next week? You might have to declare it.

I can tell you what may happen if you need to declare it but do not do so. In the departures area of the airport, you will run across one of the UK's currency sniffing dogs, and it will alert on you. Because you haven't declared it, if it's over the limit, the currency may be seized and you will have to go through a rather annoying process to attempt to get it back. But you could also be fined £5,000, which is more than half of what you're carrying.

If you do fill out the declaration, you drop off a copy of the declaration at a drop box in the airport, and keep a second copy on you. You then show the declaration form if asked. You can fill out this form at the airport, or fill it out online and print two copies before you travel.

You may also find it helpful to carry documentation of the source of the funds, such as salary slips and bank statements, in case there is any question. If customs officers have reason to believe the currency is connected to criminal activity, they may be able to seize it anyway.

What happens if you carry more than EUR 10,000 and fill in the form as you should is that it will be processed and archived by the authorities and might trigger an investigation into the origin of the funds (in the UK, I believe that HM Revenue & Customs and the National Crime Agency are in charge of this, other countries have similar agencies). If you obtained the funds legally and can document that (e.g. with payslips and bank statements showing that you saved the money over several months/years), this should not be a concern. In many cases, nothing will happen at all and you won't even have to show that (and I suspect that for a single transfer just over the limit, that's the most likely outcome).

Note that the same could happen if you make a large deposit (or a series of medium-large deposits) at a bank, etc. The whole idea behind these rules is to fight money laundering. People who obtained large sums of money from criminal activity or failed to pay taxes as they should will want to somehow get the money back into the regular financial circuit to use it and that's a good occasion to catch them. Restrictions on cash transfers (or, in some countries, cash transactions) are designed to make it harder to do this legally.

But as long as you can show where the money come from, there is nothing illegal about transferring large amounts of money in and out of the country (whether by bank transfer or in cash) and no additional tax implications or anything.

Wire transfer. Carrying large quantities of cash by hand anywhere, especially across international borders, is utterly idiotic unless you're doing something illegal with it where documentation would quickly lead to getting caught. But documentation is exactly what you want here. So do a wire transfer, pay the (very reasonable!) fee, and have nice documentation of the transfer on your bank statements on both ends.