Brexit or no Brexit you will need migrant labour to do season agricutural work, there are areas of the country where you can't gert local people to do it. Not everyone wants to work in polytunnels for 60 hours a week for 6 months a year, its good money but you have to work hard
Its the same situation is most parts of the developed world. I was in Taiwan recently, they use imported Phillipino labour
I guess it will ultimately drive the cost of food up until further mechanisation takes over doing away with yet more jobs