Cool Britannia bills itself as the biggest and by far the coolest souvenir store in London and the UK. I would be hard pressed to argue. Taking advantage of its prime location in Piccadilly Circus, it is truly one of the biggest souvenir shops in London. Even I took advantage of their massive inventory by purchasing key chains to double-decker bus shaped cans of tea during my last night. Best of all many of the items are not overpriced when compared to a similar shop in Leicester Square. On the other hand shopping outside of Leicester Square and Piccadilly Circus is likely to yield some better bargains.
The shop is situated on two levels with a cafe and entrance/exit to the Piccadilly Tube station on the lower level. Also an attraction is a member of the store staff dressed up as a Beefeater, one of different characters, in front of the store and mini coop decked out in British insignia.
My recommendation is to shop here for reasonable gifts only after you've visited the major attractions in London and could not find something to your liking or within your budget. For example, official merchandise from Buckingham Palace may cost a bit more but it'll feel more like an authentic souvenir than some of the merchandise here.
The shop is open well late into the evenings so visitors are able to shop after a West End show, dinner or a visit to Piccadilly Circus late at night. More info located at coolbritannia.com.