"to see if such a route would be viable (probably won't be, a similar route by them has failed before and Manchester didn't work for them last time)".
Interesting LAX LHR
There can't be too many airlines that meet both criteria. I'm not aware that CX, or MH have tried transits before. Have GF?
One possibility I guess would be AI, who flew to the USA and Canada via LHR I recall. But wouldn't they use BHX as they already have a scheduled service?