I don't get this -- bonuses and perks are not part of salary as far as HMRC is concerned in any other walk of life, so their salary is their salary. It should be very straightforward to measure.
Admittedly this would be their declared salary. But as above, they aren't breaking a law if they mess with the salary cap.
I have no clue - but if I was a "big" player I'd insist on being paid via a ltd company and funnel all my other benefits and activity through there. So I assume something happens along those lines.
“Salary cap” is perhaps a misnomer. “Remuneration cap” would be more accurate.
In a not unrelated subject, as a recruiter our fee is a percentage of the guaranteed earnings. In most cases a bonus isn’t counted towards our calculations unless it’s guaranteed - to stop somebody paying £1k salary and £49k “bonus”.