Yes, I spoke to one of the senior guys at West Drayton (now shifted) isn't looking forward to the VLJs.
ATC can cope with a lot but it's the London TMA that's pretty busy.
A VLJ in itself isn't a problem. However, Lon ATC is barely coping now with the amount of "pop ups". So too many more will over load them and lead to increased slot delays.
A pop up comes from places like Biggin, Farnborough etc.