Can only agree with BasilFawlty
, that some younger 767W's would fit perfectly into the Jet2 fleet. LCC's tend to stick to commonality - i.e. Ryanair stay Boeing, Easyjet stay Airbus. It would only incur more costs to have a mixed fleet - new pilot training, new engineers, new spare parts, cabin crew re-training...
The 757 is often favoured for its range of missions - short to long haul ops, and a 767WL could certainly do 4x Hops to ALC/PMI etc. one day, and fly to EWR the next.
Also, as far as I am aware, doesn't the 757 and the 767 have a common type rating???