Well, since Envoy isn't F, so technically BA is the only F option on the route.
LAX
Isn't it that BA LHR-PHL flights are now operated with Mixed Fleet F/A's (minimum wage employees on temporary contracts). Mixed Fleet service standards are awfully low and US Envoy would then beat BA F hands down. My few recent trips to LHR were in Envoy and I was impressed with each and every flight.
If you choose BA, pls check the BA forum for current Mixed Fleet routes.