It could be more beneficial to buy your dollars in the UK rather than when when you arrive in the US. This is because the pound-to-dollar exchange rate in the UK is can be better than some of the rates offered by providers in America. Some people also find it more convenient to land in the US with dollars already in your pocket – exchange vendors in airports often offer poorer rates, and having the money already available will enable you to get transport to your accommodation as soon as you land, without having to hunt down an ATM.