Thanks all...but if I'm not mistaken, the cardmember pays the forex fees, not the merchant.
Also, I had a friend in London try it. She added her UK credit card to my account online and it wouldn't take. I had also tried my SPG Amex after calling Amex and adding my friend's UK address to my account as an alternate address on the account.
I'll just buy a Top Up card or visit a Three store when I get there...