Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)

Would using a UK-based travel agency (either on line or brick-and-mortar) also work to avoid a Forex fee?

But it's hard to believe that a roundtrip fare on BA would be less than the one-way on Icelandair. (Of course, the OP might prefer to fly BA nonstop, but I expect that he will pay substantially for that privilege.)