Not quite, it's illegal but not enforced. If you get inspected within 5 years of a relationship like that you might be legally liable to repay all of your ZUS, health contribution and taxes.

And a change like that will likely come, there is a reason why all invoices are now digitalized and reported, most of fake employment relationships are one SQL query away from being found out. Just find people who submit one invoice a month to one client.

And I thought that Polish IT contractors were hired into the UK because they're good (they generally are).

You in general don't work for UK companies directly because of the laws around contractors, you have to have proxy companies to skirt the UK law.