(Peterborough Census Data) [https://www.peterborough.gov.uk/council/about-peterborough/census-2011/]

(Specifically Ethnicity) [https://www.peterborough.gov.uk/upload/PDFs/AboutPeterborough-Census-EthncicityIdentityLanguageReligionKeyStat.pdf?inline=true]

Census data from 2001 vs 2011. Check the %age of "white other white" from 2011 to 2001.

Presume minimum wage is not violated as that is a crime, if farmers want to reduce overheads they hire people who work hard.
This is where a lot of Eastern European labour comes in.
It's the same in all other markets. Simple free market economics. You want reduced costs? Hire people who work harder and put up with worse conditions.

Honestly makes no difference if they're from Devon or Poland.
If they are only there for seasonal labour it drains the wealth from the area. Shit loads of studies have been done on that, on rural to city migration during industrial revolutions. It is the same effect.

Add in the UK government only started tracking short term workers in 2016 (source) [http://www.bbc.com/news/uk-politics-eu-referendum-36271390] and you basically have shit all formal studies done, which is a travesty as if you don't you can't have proper social planning.

Visit Peterborough and see what is going on there. To be clear, there is absolutely nothing wrong with the migration that has been going on if there is social planning. The people are nice, hard workers and a net contribution to the economy. Which the government has fucked up. Thus, a whole bunch feel marginalised.