Originally Posted by
Shuttle_Endeavour
I would go for National Gallery and London Eye. I assume attending a service at Westminster Abbey is free of charge, but if you visit outside of a service beware the entrance fee is close to £20 per person.
London Eye is a good shout, and then a wander down the south bank or have a drink around there. Services are definitely free of charge, but arrive in good time if you want to sit in the Quire.