HI,
I took cash and I bought a post office travel card, you 'top up' before you go and it can be used as a visa card while there, it was really handy because I could use it as a debit card and could also draw money out from a ATM machine if necessary. Travellers cheques are probably the other option.
Im guessing the flights will be the same route, on the way out we flew to San Francisco (11 hours) and a connecting flight to Las Vegas (1 1/2 hours) I wont lie it was hard work but worth it! Coming back we flew from Tuscon to Washington (approx 4 hours) then washington to Heathrow (7 hours) it was much easier coming back the little time at washington helped me anyway!
You probably will feel the jet lag when you arrive in Las Vegas, obviously it effects people more than others but it subsides after a couple of days! Coming back wasnt to bad, I was tired for a couple of days but this was probably due to a wonderful holiday!
Donna x