As fl dutchman said, the introduction of domestic Club Europe has hit the number of available seats on the LHR route. In my experience, there are normally at least 5 rows of CE on the NCL flight which is 10 less seats overall as the middle seat is left empty for business class passengers.
For an A319 this is a 7% reduction in capacity and for an A320 this means 6% less sellable seats.
The winter timetable does not help loads on BA as the first flight out of NCL leaves too late - if you are connecting on to a European destination it is far more preferable to get a 6am KLM flight than a 7:30ish BA flight. Heathrow slots are probably the issue here as for the past few years the winter timetable has a far later first flight out of NCL than in summer.
BA are retiring the 767s as well so that will necessitate some shuffling around of the A321 fleet and a knock on effect for lower yielding routes.