I understand it might seem logic that more crew is available during the Christmas holidays, but it is actually not the case. Crews deserve a rest too, and once the production plan is finalized the necessary crews will be planned (including the normal operational reserves). If you need crews for multiple extra daily flights during a whole week, you'll have to call a lot of people that are currently on holiday... also, one crew will not be able to make 4 rotations a day, and the positioning flights in your plan make things even more difficult.But many airlines have been reducing their capacity because of lower demand and because of winter.
So,I guess finding some planes and crews to operate a 1 week shuttle can not be that difficult.
1 crew could make 4 rotations a day on these short Channel hops,...or not?
It also depends on how "unionized" your airline is. At some airlines, people would be more inclined to provide their employer with some extra flexibility compared to other airlines. Unions need a reason to exist too, so at some airlines they might occupy themselves by checking all the different crew pairings etc. to be really sure that every minimum turn-around time is respected etc. etc.
In short: one or two ad hoc flights (like JAF and FQ between BRU and LON) is possible, but nothing structural.