I want to travel a bit

A friend of mine just told me there are cheap airline-tickets on sale from Saint-Petersburg to London.
I've never been to London so I want to go there and check it out.
I already have a nice travel guide

Now I'm trying to find out how much does it cost to stay one day in London.
Do you know?
Also I'm looking for a good and a cheap hotel.
Do you know one?