What is the best strategy to exchange money to GBP in London?

Unless you're changing a huge amount and/or your time is worthless, just pop to the nearest non-touristy high street and compare the rates at the banks, the Post Office, travel agents, and maybe Sainsburys or Marks and Spencers (or a selection thereof). If its a very small amount just find the nearest reputable branch of one of these and be done with it.

If you want to find the absolute best rate then by all means use MoneySavingExpert which is a well known and well regarded site in the UK, but bear in mind that some of the offers listed will have to be done by post or are companies without many branches. The site's general advice is good, however - specifically, avoid changing money at airports whenever possible!

Another option is to hold on to your spare currency and change it at home where you know the system better or change it at your next destination.

Finally, try not to accumulate too much cash in the first place. I travel with foreign currency cards and do as much of my spending on the cards as possible. In the UK its possible to do the vast majority of spending on a card.

IMPORTANT NOTE: MoneySavingExpert do not offer the services listed, they merely provide you with market information. Note their disclaimers: "If these companies go bust and have your money, there's no protection. Don't let it have your cash any longer than needed. Inclusion here does not imply solvency."

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.

Here in Switzerland a lot of banks accept EUR as deposit as well. If that is the case in the UK, you can go to any bank office with a deposit machine, and then deposit your euros back to your account, probably at the exchange rate your bank offers, showing up as GBP on your balance sheet.

This will not only convert your money back to your usual currency, but also puts it in your bank directly, thus preventing you from having a large amount of cash for prolonged periods of time.