I'm a little late as you've made the call now, but just to add my 2 cents.......
Until a few years ago, BT didn't allow calls to U.S. 800 numbers. (It was possible to call them if you knew the correct operator bypass codes, but of course they weren't handed out to the general public!
).
You can now call any 800 number (or the newer 888, 866, etc. toll-free codes) by dialing normally and will get the message (from the American end) telling you that you will be billed at normal rates, as mentioned above.
I think certain 1800 numbers are not enabled to work outside the US
Certain 800 numbers may not be available outside a particular state or geographical region within the U.S. The owner of the 800 number can specify from which areas he will accept calls (he's paying for them, after all), so you might find 800 numbers which work only within, say, GA, AL, and FL, or only accept calls from west of the Mississippi, or accept all U.S. calls but not Canadian, and so on.
In the specific case we're talking about, the
caller is paying from the U.K. end, of course, but unfortunately whether the call will go through will still depend upon how the number has been configured at the appropriate central office. You may even find that you can call a particular 800 number through one carrier but not another.
If the call is blocked, you'll normally get a recording which says "The number you have dialed is not available in your calling area," or something to that effect.
By the way, some of the largest American mail-order companies, banks, and so on now have a British 0800 number which routes directly through to the U.S. It's worth asking!
