The UK based teams will have to return tax records, which I assume the auditor will have access to. This is a very good 'cross check' v what is supplied to FIA, but unlike the tax people they can not directly cross check with the supply companies. As I said previously, these people do it for the governments and law agencies to to catch crooks by the back door, so it would tabe a brave or foolish team to deliberately try to pull the wool over their eyes. Not only now, but if something comes out years in the future it would not be welcome.
Accidental overspend should probably be expected if not accepted but any attempt at deliberate cheating should be stamped on hard
Edit.
Something that had not occurred to me is how would they treat volunteer workers and work experience (by any other name)?
Some 'students' doing unpaid work experience with the promises of consideration for a job in this or other owned companies can cover a lot of labour and skills, especially if data processing is considered part of their education not team development.
I would think all the teams would have options to fiddle this if it was not expressly controlled.
We did touch on it long ago, but no one seemed to know much about the reg.