Just be warned that this is the tourist season, so at the popular places like The Tower and Westminster Abbey, you can be in for long queues. And although there are exceptions (most of them expensive and fairly well hidden) eating in the West End is a bad idea.
A couple of places (if you like the food involved) would be
Selfridges' "Brass Rail" for a lunchtime salt beef sandwich and an amazing Thai restaurant (despite looking totally unprepossessing from outside)
The Blue Elephant in Fulham. They even have displays of orchids in the washrooms
The "Rough Guide To London" is worth buying.