Given the reduced number of destinations available, I'm surprised services to places like Mykonos haven't been stepped up. J2 only fly from MAN twice a week with hefty fares. Santorini only 3 times a week. Are these places slot constrained ?
The constraint is far more likely to be accommodation - or at least affordable accommodation - as neither JMK or JTR have endless bed stock. I'd be surprised if slot constraints prevented more flights, not least because the aircraft aren't working as hard as in a normal summer so there would likely be flex in the schedule to accept whatever slots were on offer if you had to.