brussles airlines operate lgw-bru
In total for London then:
Brussels Airlines from Gatwick to Brussels.
BMI from Heathrow to Brussels.
British Airways from Heathrow to Brussels.
VLM from London City to Brussels and Antwerp.
British Airways code shared on LGW - BRU with Brussels Airlines up until recently. I think when the code share stopped the frequency reduced.
VLM's Brussels route is down to one daily.
BMI are now using much smaller aircraft.
Blue Air recently stopped flying between Stansted and Brussels.
Ryanair's relaunched Stansted to Charleroi didn't last very long...
So yes the London to Brussels market has seen a reduction in number of seats since last year, but maybe this is a natural adjustment.