It's not quite as bad as that - looks like the corresponding flight is BA239 which leaves LHR at about 7pm and arrives BOS at 10:30pm so about 9.5h for the RON. I've taken the eastbound quite a few times and the load varies but can be reasonably full. The late westbound is probably the alluring part as much as the eastbound - it allows people to finish a day in London, and can scoop late(er) connecting traffic from the continent.