assuming flying, not driving, long distances, I'd fly into Boston, take train or bus to NYC, fly to Vegas, fly to Orlando, then back to UK. (BOS-->NYC too short to go through airport hassles). I am in the US and have traveled between all 4 cities. Vegas back to UK would be a long flight hence recommend doing Orlando last. Doing the whole thing in reverse would also work!
dr b is right but generally a return fare to one place, whether Vegas or Orlando, is a lot cheaper than single tickets to one and back from the other. Driving in the States is actually enjoyable but the distances are indeed big, especially going to Vegas. ( If you are in Vegas, you're most of the way to California, too.)
Thanks all, hubby not too keen on driving as we havent been in us before . Although not going till end of year will be booking flights soon ,good to know best way to go Thanks again