I have found that the airport exchanges are the WORST to go to, so avoid those.
Local banks are hit or miss. Some have good exchange rates, others not so much. I guess just do your research on what a "good" rate is and then see how the bank(s) compare.
London has many "money exchanges" set up in touristy areas. Like banks, these are also hit or miss. Some have good deals, others do not. Again, just knew beforehand what the rate SHOULD be and you can identify whether it's worth exchanging or not.
Some hotels will also exchange currency.
In my travels to Africa (specifically Mali and Gabon), I found that the best exchange rates I got were at local restaurants. Not all of them exchanged money, obviously, but I found a few who did and they gave me much better rates than the banks did. For example, the "official" exchange rate was around $1 USD = 480 CFA but I was able to get $1 USD = 500 CFA at some places since it was just easier for them to give me 10,000 CFA, rather than 9,600 CFA, in exchange for $20. This may or may not be possible to do somewhere like London, and these kind of places likely will not exchange large amounts of money either. (In my case, the most I ever exchanged at one time at a restaurant was $60).
However, if dealing with a non-official exchange (official being a bank, at the airport, etc.) then be careful and make sure you are actually getting "real" money instead of counterfeit.
Alternatively, if you think you might go back to a Euro country within a year or so, you might just want to save it. You generally lose a little during each transaction, so it might be better to just set it aside for your next trip instead of exchange it to Pounds and then exchange it back to Euros again.