If any Britons wanted to work for the wages that immigrants take home, those low-paying jobs would be theirs immediately.
That's a terrible framing of the situation. The opinion (rightly or wrongly) is that cheap labour from poorer countries sets the expectations for employment costs.
These workers will come for a season and live in horrific conditions with the expectation that although it's low pay for Britain, its higher than the home country and they will take the cash out of the economy.
Brexit was a sledgehammer approach for people to say "no jobs should pay so low that you have to live a subsistence existence."
Now whether it's because farmers pay too low due to the supermarket purchase chain forcing the situation, cost of living is far too high or a mix of both, the fact of the matter is just saying "tHoSe LoW pAyInG jObS cOuLd Be ThEiRs" make you sound stupid.
There are a lot of problems with the British economy and even with the elections coming up, the opposition are not coming up with very good suggestions on how to improve the situation.