Nigel,
Buying film once you are here in the USA might be the least expensive way to go and you only have to worry about airport security on the return. One caveat: unless you will be in a city that has a good photo supply store, finding film may not be easy and probably will be at least as expensive as film in the UK. B&H Photo Video in New York is an excellent place to save money on film, for example, and, if you have or know someone who has a permanent US address, you could save even more by ordering in advance via internet from B&H, Freestyle, or several other vendors.