In days gone by you could zig zag down from Tottenham Court Road to Leicester Square going from bookshop to bookshop. With Borders having closed the zig zagging is now slightly more spaced out with Foyles then Blackwells the places to start from.
Blackwells still has a feel of being a bookshop targeting academics but in terms of choice it is hard to beat it and hard to come out without having spent a fair bit of money.