A lot depends on what you are starting with.
A B1 licensed engineer with a type rating and an existing C licence will only require type training including practical and OJT. Say 8 weeks training max to get the B1 ( with the C being automatic) on type.
A B1 basic holder would take about six months after type training with the C taking a further 3 years.
The C is a nice to have but not a necessity for a line operation.
I have seen various figures including one as low as 8 for the numbers of engineers required..
I would say that is not far out but leaves no flex for leave, training or sickness.
It could be covered with extra staff or a floating pool, either will work.
Would the CAA give out licenses to ex military staff?
No would be my answer to that. The CAA already broke EASA regulations by granting licenses to employees of British Airways who only held American A&P qualifications. That has caused so much trouble that I doubt they will repeat the error.
Every engineer working on SAR has known this was comming so I would imagine that those keen to continue working in this area are already well on their way to getting their civilian qualifications.
Most smaller offshore helicopter operations in the UK Humberside, Norwich, Yarmouth work with close to 100% licensed engineers. Mechs are few and far between. Increacing numbers are dual rated B1 (airframe and engines) and B2 (Avionics).
"Real" B2 engineers are really like hens teeth and that is where the winning operator will struggle.
There is a lot of interest in these positions in the wider helicopter community and I think that the positions will be filled without major grief. In addition these jobs will be open to all EU citizens so a wide pool to fish in.