Easy have taken quite a bit of the leisure market on London to Inverness. The BA route retains much of the business traffic, mainly due to its better timings morning and evening. It is normal on a UK business route for Saturday to be the quietest day and to have a reduced service. I would guess the BA loads are thin on that day nowadays.
The days when the full service could be maintained at weekends was before Easyjet and BMI came onto the route. There has actually been a huge increase in London to Inverness capacity over the last couple of years.