Thing is mate, that's not illegal. Tax EVASION is and so it will all depend on how its all been done. People have set up their own companies in order to pay in their earnings to avoid PAYE and any profits declared are then subject to a much lower tax rate. eg 20% not the 45% they would have to pay in as PAYE. But there is the black economy where traders work on a cash only basis and so nobody knows what their earnings are.
I only worked for BR and so had no choice.
But then companies can declare any profits in the Cayman Islands as well.
Its a minefield for the inland revenue. I would think many millionaire footballers are looking for the same avenues even though they will only ever spend a fraction of their wealth or earnings. One way is to tax them all at source like PAYE. Maybe that's the only way. Individuals and Companies alike. Including sponsorship contributions and other non football income like after dinner speeches and fashion wear.