Changing Money in London Banks

A lot of banks only do it for account holders these days. You can definitely change that money at the airport at one of the exchange desks.

Go to a specialist currency dealer - you will get a better rate than bureau de change inside banks. Thomas Exchange Global is one that has been suggested before but there are others. Usually makes sense to get a voucher from their website to fix the rate. If you have very little left find one near where you are or going.

If you plan carefully, you could just use up your euros paying all or part of your hotel bill. Then you wouldn't have to exchange anything.

I find a way to use up my Euros before I travel to a country that has a different currency, either using them to pay down my bill from wherever I am staying or taking them home to save for my next trip to Europe. If you decide to use an exchange service, be prepared to get a poor exchange rate.

And they probably won't exchange coins or small bills.

In addition to the specialist forex dealers mentioned try somewhere like Marks and Spencer (in most of their stores) or the Post Office. The rates won't be brilliant but better than the banks. Small denomination notes should be ok but no coins accepted.