I know the Bourgas flights do very well, but I reckon the high frequency could be partly explained by the drop in capacity offered by the charter airlines this summer.
There'll be no more Monarch from Gatwick this summer. Thomson is decreasing capacity from 767-300s on the Gatwick route, to 738s/752s and from 757s on the Luton services to 738s too.
This seems quite drastic to me considering theres no more FCA, Astreus orViking services either. Also TCX pulled out of LGW-VAR last year I think.