Ski, you're quite correct. You'll need to dial 00-1-205-###-####. 00 is the code to access international dialing when you're in the UK, and 1 is the country code for the US. (BTW, Scott and Dawn, while you two are technically correct, that's only out of luck -- the national long distance code happens to be the same as the country code for the US, but that hardly ever happens anywhere else, so it's not the best rule to rely on when trying to make international calls.)
If she wants to call you, she'll need to dial 011-44-(your area code and phone number). 011 is the code for international dialing in the US.
For future reference, you can look up a monstrous chart of international dialing codes here:
http://kropla.com/dialcode.htm
And don't forget -- international calls are expensive!
EDIT: Also, not all mobile companies will let you make international calls... you may want to check first. (=