Like every other route they have started, it's summer only to gauge demand. Once they see the demand is there, then they will try and operate a winter schedule too.
Hang on, that's not the case with United (ne Continental).
All their UK regional stations were launched as year round from Day One with the exception of Newcastle where they struggled to do well in the summer peak, perhaps confirming their initial preconceptions. This one's been an oddball tbh from day one.