try this website, is great, you can found all the details you need.....
http://www.visitlondon.com/
you have to think about what kind of things you fancy to see, cos, you can spend the whole holiday just going to museum and gallery.....
or shopping......
food ma, fish and chips....
from £20-£50 a day, not shopping ga.....
weather..... this Aug is very wet...... not much sun....
this website will tell you more....
http://www.bbc.co.uk/weather/