I wouldn't be expecting any major changes on the LHR from BA. Just like the last time they got the route to themselves, I will really only be expecting to see a slight upwards shift in capacity - I'll be surprised to see anything other than minor changes in frequency.
Also bear in mind that the A321's will soon be configured with more than 200 seats - not too much less than the 767's.
Oh, and the pax numbers actually increased when BD were pulled off the LHR.