We now have a tarrif free trade in agricultural goods with the EU. So UK wages go up, uk prices go up and continental food becomes way more competitive so we import more from the EU. "British" jobs still go to low cost East Europeans, just in the EU and not in the UK.
Experience suggests that most consumers won't buy british if it costs more.
Maybe in the long term we'll find new niches, growing higher margin more perishable crops with more mechanisation. In the short term it will be the "destruction" part of capitalisms "creative destruction"
I don't think this can happen in services so easily.