If a 757 has 50% more seats than a 737-300/700, then a turn around time of 40 -45 minutes (25minutes + 50%= 37 minutes) would be equivalent.
Britannia used to turn 767-200s in 45 minutes, and those aircraft had 273 seats
If Jet2 wanted commonality they could get 737-700/800s on the used market, could they not? Those aircraft would have the range to fly 4 hours out of LBA. They would pay more than the cost of very old 757s, but the newer planes would have advantages.