I am not a big fan of chain restaurants and in London there are plenty. I did however enjoy Le Pain Quotidien at the Royal Festival Hall and in Marylebone. I think it is more of a breakfast or coffee/tea place rather than for dinners. They do serve dinners as well, but the food is not exciting.
It is a tiny bit of the expensive side - perhaps around 20 GBP per person (they add the service to the bill).
They have nice fruit salads and a bread baskets with a large selection of spreads (I love the blandis and chocolate spreads).
The setting is casual and since there are not that many small tables, be prepared to share a communal table.
Belvedere Road
Tel. 020-7486-6154