It's really, truly not. He will pay the same rates of PAYE and NI as any other UK-based employee, and moreover Chelsea will also pay 15.05% employer NI on top of his salary each month.
Overall, each year >£7M will be going to the Exchequer as a result of GP's £12M salary.
Is he 'an employee' though. Or is he a consultant/contractor type relationship? In which case, he may not be subject to full PAYE etc.