Not 100% sure but don't think any US carrier will allow you to book on their sites and pay in $US. You need a US address for the booking. Most if not all of them have a UK section to their site and you have to pay in £UK's.
One way that it is possible is use Orbitz(must be others also) I have a free flight out this August using Virgin Miles needed a flight home, used Orbitz to book a BMI flight at $450 each, direct with BMI it was £450 each.
Hotels I have booked a couple of times with US sites but in general UK sites get better rates if you use sites like Hotelopia, Alpharooms or travelrepublic as there prices include all taxes and fees, US sites are usually just a base price(but not always) Hotelopia has been great this year with codes around upto 15% discount and 10% cashback thru rpoints or Quidco.
For tickets we have AP's for all parks and pay with Nationwide cc. need a few tickets for friend at Easter, bought them all direct from the parks in $US, they have good offers on, not worth using an agent.