Oh, and do I need to mention that the jobs aren't year-round? If you want to work all year, you probably need to be able to travel, which is going to be more difficult since brexit than it was before and the pay certainly won't be as good everywhere. In some places, you aren't even afforded the same protections under the law because you are a temporary migrant worker.
When you simply quote the amount paid per hour, you cannot get the whole story here. And that's assuming that your £30 an hour quote is a flat rate per hour: I don't know how it is there, but in many cases, farm workers get paid per piece, so it might realistically be "up to £30 per hour" instead.