It would be interesting to learn how much business from secondary airports in Europe and the middle east uses LHR to hub on to other (BA) services, or in the other direction with passengers from secondary north American airports using BA to hub into Europe.
It's still an incredibly strong proposition to market going West.
Even United who have a STAR hub at FRA have more flights out of LHR due to the much larger O&D market which also allows a swathe of connections onto LH FRA flights, I kid you not.
BA/AA is also an incredibly strong offering. Fed from both ends, they offer 23 US airports totaling 61 flights per day going into the USA.
In total it's :
Oneworld
AA 20
BA 41 US + 5 CA
STAR
AC 10
UA 17
Skyteam
DL 12
VS 17
So around 122 westbound scheduled daily transatlantic flights. Still #1 in that market by quite some way.