If u can find the local money exchange stores in your area guving u a decent conversion, that is the best option. Exchanging dollars to pound at the airport or using dollars in London will hit u with worst exchange rates. If u r planning to spend few hundreds, just use the dollar there or exchange at the airport. But if you are planning to spend in Ks, using dollars or exchanging at the airport is not a goid idea. If u live in the city area, google it and you will find lots of money exchange agents.