Oh I'm sure it isn't optimal, but I can't help but think 18.5h utilization requires absolutely everything to go to plan, with zero delays and perfectly timed slots for slot-controlled airports such as LHR. In the case of an east-coast to LHR transcon, that's something like 3-3.5h on the ground from wheels down to up on each leg. The more traditionally-timed departures (early evening USA / noontime UK) probably come close to supporting that, but I think it's really more like 4.5 hr each end inclusive of catering, cleaning, cargo, etc.

I based my timing on Flightaware's tail number tracking which seemed to show aircraft pairs pingponging between BOS and LHR, but they might just increase utilization if the frames fly another eastbound leg before turning back west. Then the 9.5h gets amortized a bit differently. But I don't know if FA's data is accurate or not.