If every airline planned their operation around what they’d do on a cold dark Wednesday in January, nobody would bother getting into the industry.
The fact of the matter is that airlines make their money in the summer to get them through the winter and Jet2 are exceptionally good at making that summer season longer (notice flights to the Greek islands, for example, now extending into the second week of November) and making the winter more productive. Christmas markets, Iceland, ski flights and charters for other operators.
This hypothetical 9th (or is it 10th?) based aircraft would be busy March-October and they would find use for it somewhere in the network over the winter. The aircraft are coming, they have to use them somewhere.