Plus airlines don't consider paying training for pilots anymore necessary so I just feel its highly unlikely they would be willing to train untrained pilots during a possible tight pocket period unfortunately.
But how many many completely untrained pilots would the airlines actually need to pay for these days? Just like now post Brexit people will still shell out to go to the likes of CTC, especially if they think their employment prospects are improved.....
It would be interesting to see how many ATPL holders we have in the UK currently stacking shelves, unable to gain employment in an airline. There will then be the pool of those perhaps fully qualified in a previous airline but laid off ( not sure there's many at the moment)...and then of course there are a fair few fully trained UK expats who might head back to these shores if T&Cs were suitable.
In reality I suspect given the planning that has gone into Brexit
I think what will happen is at the 11th hour somebody, probably an airline CEO, will pop up and say:
" BTW what are we going to do about airline pilots and visas/right to work? I need to replace 100+ this year, the UK passport holder pool is sufficient but the experienced ones can be a bit pricey to attract, we need to keep fares for hard working families down, and of course we need as large a pool as possible to select from so we can maintain standards etc etc "?
There will then be a bit of a "Ooooh" from someone in Government , and as you say the UK will have to sort something out re visas and/or with the EU at short notice....What the arrangement will be is anybody's guess.