Just to add to the post.Here is the "official" guide to London,with interesting links of places to visit and things to do:
http://www.visitlondon.com/
Bare in mind that the UK weather on the whole,is as unpredictable as a hot potato,but any time from late April onwards,and you should be OK.It's a lovely city.One which I've been too several times.Just make sure you bring a currency converter,so you can judge how much your getting for your dollar :D
Oh yes,and please get an underground tube map.It may look like a bit of a nightmare,but it's the easiest way to get around.