now that you mention it, all the places I used to go to seem to have shut down.
The Tridias shop has closed down, there is only a branch in Richmond now (also Bath and Dartington, I see from the website).
I also used to go to Neal Street East and get loads of stuff, that has closed as well.
Some other ideas: Muji (especially for stationery, little folding toothbrushes, that sort of thing); any shop specialising in tricks and magic and practical jokes (there used to be one just near Russell Square);
will you be anywhere near a branch of The Natural World?
stocking fillers
list of stores