I was poking around on the website last night trying to find standard coach seats SEA-XXX-LGW in May... very slim pickings... then it occurred to me to search LHR, and I found a long list of transat flights avail for 25k each way. It was a weird, unaccustomed thrill to see all those blue buttons appear under the "Standard" column...
I got all the seats I wanted for a long weekend in mid-May, SEA-IAH-LHR-EWR-SEA, for 50k RT, and I think there must still be plenty out there, so go for it. Maybe people just don't think to try LHR yet. Cheers.