I think they've got a kiosk outside the north entrance of Gower Street Waterstone's on Torrington Place (couple of streets off Tottenham Court Road behind Habitat) as well.
I had a cappuccino so can't comment on the espresso in isolation but the prep and pull for the cappuccino were good with a bit of latte art to finish.
As charmon said we're becoming better/well served in London, nice to be able to pick up a decent coffee in passing from a street kiosk on a cold afternoon.