Destefanis

It is a common thing in London to see shops in touristy areas charging exorbitant prices for tiny, poorly made tchotchkes. A post card for 4 pounds? Seriously? The owners are often overly agressive and generally unnerving to deal with. I personally avoid Oxford Street at all costs for this reason.
Amongst the slew of garbage souvenir shops to be found in London, Destefanis is a gem.
Relatively unassuming from the outside (where postcards are on sale for 1Pnd, 50P, and 5P (!!!)), the shop is filled with reasonably priced trinkets, post cards, knicknacks, sweets, newspapers, and magazines. The shopkeepers are sweet people who are friendly and helpful, and very pleasant to interact with.
The next time you need a tiny red bus to bring home to a friend at Christmas, Destefanis is your best bet.