I'm not a 100% sure but I believe Icelandair's BC seating is configured more like U.S. domestic FC rather than like int'l BC. It's not going to compare well with most trans-Atlantic BC seating. Not unreasonable at FK's longest route is only 3,530 miles or so (KEF-MIA).
LHR-KEF is about 2 1/2 - 3 hours so is the premium worth it to you?