Paul,
IB won't let you change the base currency of an account. So if it's £ now you can't change to $. However, if you want to trade $ listed futures or stocks you just do it. They automatically create a position in $ which covers your position. It has a margin requirement, and you will pay interest, but it is effectively seamless. I have a £ IB account but I only ever trade in euros at present, there's no problem. If you want to move winnings to £, or cover losses you just buy or sell them.
If you are only ever going to trade in $ then a $ base account may be more effective, but you will have to cover exchange issues when you fund the account, or withdraw funds.