The problem is that you can be full of low yield traffic and still make a loss. The break even load factor of economy in low season can be well over 100%.
A number of UK - USA routes outside London have been started, achieved high load factors but discontinued as unprofitable.
I suspect that the problem with LGW - NYC is critical mass. I remember British Airtours operating LGW - EWR twice weekly with a 707 to keep a competitor off a route such was the lack of demand for EWR. Now demand is high. If a major operator wee to open a hub at LGW demand would soon appear.
Not likely to happen though.