AUK don’t need to make an additional rule. It is illegal and that is enough. AUK can already refuse to validate brevets that are ridden illegally.
The problem there is disproving it when the insurance come calling say:
10 riders arrive at a cafe together in North Berwick, 5 provide a receipt from the Café till after placing their order, the other 5 got to the cash machine round the corner after eating.
5 miles down the road 2 riders clatter in a heap on the road and the police and insurance company gets involved
In that scenario the insurance was invalid and they broke the law, no pay out and potential immediate cancellation of AUKs insurance with all that entails.
If they hadn't crashed AUK would have been none the wiser.
10 riders arrive in North Berwick, 5 in 1 group, 5 in the other, the first 5 go to a café, the other 5 got to the cash machine round the corner.
5 miles down the road group B overtakes group A, two riders get tangled up and the police and insurance company get involved.
1) Prove to the police this isn't an oversized group based around a single event (after all, they're all carrying the same brevet card)
2) Prove to the insurance company they wern't riding in an oversized group based around a single event and therefore that the policy was valid and shouldn't be summarily terminated.
The only thing protecting AUK from that happening is limiting daily entries to the maximum group size
It's also easy to say all the riders would submit their GPS trace to the police and insurance company to prove it was situation 2 not situation 1; aye a mate did that after being shoved off his bike by a reprobate on a scrambler.
The polis weren't interested, so unless you're going to contest the fine to the Sheriff and take the insurance company to court over the annulments, both of which will cost you more than the fine itself it's really not worth the effort.
CTT events can confirm which entrants were on course in their entered capacity because there's a time keeper, at the start/finish line taking notes and can kick them out the event for any transgression of the rules, because the whole event is observable.
Every sport is having to work out how to adapt to fit the current restrictions and insurance climate; some are easier to adapt than others.
And the more "free" the sport is in what entrants do, the harder it will be to adapt because the organizer has less control of the entrants actions.