There seems to be some confusion about whether it's possible to get through to US 'toll free' (1-800) numbers from the UK. Some people have reported being able to get through simply by putting 00-1 in front of the number but the consensus view seems to be that, in most cases at least, it doesn't work. (You certainly can't access UK 0800 numbers from abroad).
What is definitely beyond doubt however is that, if you can get through, the call won't be free. You'll be charged at the usual international rate for calls to the USA.