After Brexit I can only see practices like this increasing in the UK. A lot of the farm work is done by immigrants, because natives don't want to do the work. People aren't going to pay more for food, so the wages can't go up, meaning more 'efficient' farming practices will be used.
While your examples and reasoning seems good, I don’t think it’s enough to support the conclusion.
From what I have seen, everyone who thinks there are any significant difficulties that need to be overcome is a Remainer and everyone who is a Leaver is absolutely certain that any claim of difficulty is “just project fear” — this, combined with the inherent time lag between planting and harvesting, means that what crops are around next year is entirely down to what UK farmers already believe will happen next year.
Four outcomes, for any given farm:
1. Current crops, easy access to labour: fine
2. Efficient crops, easy access to labour: fine
3. Efficient crops, no access to labour: fine
4. Current crops, no access to labour: bankruptcy
You may consider 4. to be an obvious safe bet, but when I say I think Leavers regard any talk of difficulty as total nonsense, I mean I get the impression they place it in the same mental bucket as “Satan is real and knows your online banking password”.
In outcome 4., the farms won’t be around afterwards to try it again the right way.
Given the UK is a net food importer, this is mostly separate to any question of food security, but ironically the lack of access to labour could increase unemployment.