I'm an American, so I have no advice about how to book a tour from the UK, but I do travel a lot in Europe and I like to call home often, so I do have some advice about calling the US from the UK without paying an arm and a leg.
Go to skype.com and create an account. This is free, and allows people to call from computer to computer for free anywhere in the world, usually through a webcam so they can see each other. That's not helpful in your situation, though, where you want to call an overseas company rather than chat with a friend. So you can get what's called Skype-To-Go Numbers for your account. You tell it you are in, say, London, and then you give it the American number you want to call. It will then give you a local London number that you can call and be connected to the American number. You won't have to pay your phone company anything - it's just like calling next door. Skype will charge you roughly 2 cents a minute for the call. (You prepay by credit card in $10 segments.)
The great thing about it is that not only is it a ridiculously cheap way to make international calls, you can do it from 20+ different countries. So you can use it at home in England to call the US (or anywhere else in the world), and you can also sign up for local American numbers that you can use once you are in the US to call home for 2 cents a minute.
Hope all that makes sense! It's been an amazing help in all my travelling, as well as just calling friends long distance within the US.