Well the only trouble is that if you need a taxi from the airport etc you will need £, no one will do anything for $ here I am afraid, but you always get a better exchange rate when you are translating into the native currency - so generally I would get more $ for my £ if I took my £ to America and transferred them there.
But as I say you will need some when you get here.
I think you'll have to have a think about what it is that you want to see in London - but a good place to start is Westminster, by Waterloo station you can head towards the River (there will be signs you need to follow that say Embankment) and when you head towards that it will take you to by the Thames. If you look around you you will see a gigantic circular thing that looks like a fair ground ride, it's not - and it moves very slowly taking you high up into the air to see London from a unique viewpoint.
Here's the site
http://www.londoneye.com/about/facts.asp?section=about&secondnav=facts You also happen to be right by Big Ben and the Houses of Parliament there which are popular with visitors.
Let us know what you like - history, architecture, theatre etc and we'll suggest things
