I think the intention would be that it will eventually have infrastructure to support stuff probably up to the size of a 737 when they repave/lengthen(?) the runway and build a terminal.
Not sure about the accuracy of the 50% load information... The runway is 6000ft long which isn't much shorter than Newcastle or Leeds not to mention Bristol and they manage fine to the canaries and cyprus from there. I'm not aware of other operationa constraints in flying from Carlisle.
Easyjet would do fine, the A319 would have very few problems