a) Tesco Expresses are much more profitable (profit/area) than supermarkets
b) On the rare occasions when a large enough plot comes up (eg 1 Stamford Street), the council allows other things.
I'm quite doubtful whether Great Suffolk Street would be a good location for a supermarket, though. It's not well served by buses and I'm not sure the surrounding roads could cope with the delivery lorries.
Other use, such as a solictors (A2) would require planning permission. Permitted development allows a change down the numbers (ie from an A3 to an A2 or an A1), but not up (ie from a shop to an estate agent or a bar).