I've flown on enough B744s & B777s out of LHR two thirds empty & also flown many times in "J" from MAN at a substantially higher price than flying from LHR.
I'm not talking about a "struggling" LHR but some airlines "struggling" to fill their multiple schedules.
Any routing MAN-xyz is meant to me more expensive than MAN-LHR-xyz, that's the crazy world of airline revenue management. It's not a LHR/MAN thing more a global spoke to hub behaviour. I may be assuming wrongly here, that you mean BA-US when you say multiple schedules, the only thing I would say is that's the market the real money is on for them, so it seems to even out.