If you want to get a bit more bang for buck then I second the vote for Giant or possibly a Kona. I've had both and find them both very good bikes as hardtails go. I wouldn't necessarily insist on a road bike for London although you'll be hard pressed to find genuine need for off-road (knobbly) tires so what ever you get should have road tires on them. Also, you should look for something with little, or restrictable movement in the front suspension as popping the occasional kerb or pothole won't demand as much as off-roading will and the less movement means more power, particularly up hills.
Best thing that I can recommend is to get yourself down to the Bike Show this weekend and have a good look about. A lot of shops will be demonstrating alongside the manufacturers and you'll be able to test out a very wide range of models and possibly pick up a bargain.
If you do go, don't forget to check out the London Cycling Campaign stand. The membership is worth it for the discounts alone.