There’s three reasons why I don’t share your optimism regarding engineering being able to support CFMs en masse in the short (i.e. less than a year) term:

1. Engineers who had experience of the 737 CFM56 are retiring. Engineering, especially at Gatwick, had an age profile that resembled a cliff, and BA is going through that cliff edge. None of the new recruits have CFMs on their licence, or if they do it’s due to experience at previous airlines.
2. All tooling and equipment for the Gatwick 737s must’ve gone now; I think they were sold together with the planes.
3. The NEO engine is still different from the CEO CFM, and would require some sort of conversion.

let’s also add that the potential for wet lease, in this precise moment, is fairly limited.