In the uk you can usually exchange currencies in most major banks, lloyds, nat west, hsbc, royal bank of Scotland. However from personal experience the rates are not competitive.
I have found that travel agents (Thomas Cook, and Thomson/first choice/tui) and marks and spencer department stores with buearax de change in them offer a better rate than the banks, and are usually close in rate to each other.
Larger department stores in bigger towns and cities may offer the option to buy in foreign currency, some marks and Spencer's definitely do, but the rate will not be to your benefit.
In larger towns some supermarkets such as Asda tescos and Sainsburys now also have exchange beaurax
The best rates in my home town can be found in a couple of money lending / pawn / cash converter shops, in particular for euros and U.S. dollar exchange.
You can change money at most post offices in the uk but there are some limits. There are several types of post office, and in smaller local 'sub' post offices you can only exchange euros and dollars, larger main or 'crown' post offices are likely to offer a fuller service, and the rate is normaly a few pennies better than the smaller ones.
There is a website rue by a uk financial advice guru called Martin Lewis
You Imput the amount you want to exchange, and it will provide you with a table showing the best rates. if you are changing euros to gbp then you need to use the buyback option.
It also sometimes has downloadable codes / vouchers to get a slightly better rate (often if you are exchanging a higher amount of currency)
Final point. The most expensive place to change money is at the airport. The rates offered are often attriocious, due to having a captive audience.
Enjoy your trip.