Those kind of configurations are heavy penalized by the game in order to avoid players to exploit it.
All C flights will only (maybe) work if your Company Image is really high, there's no competition on the route, and the C demand is very high (LHR). Otherwise, you'll be heavy penalized.
You also have the "too small for the route" penalty for crossing the atlantic with the 757, so both penalties may add up and you are getting those 0% LFs.
I'd put that 757 in your short haul routes.