I think the rule for clothing should be the same as for the bikes - more or less pre-1987 stuff, with replica jerseys in modern materials accepted. This seems to be the rule in the RRVV. Nobody's forced to wear tweed, thankfully.
Helmets are more difficult, as usual. It seems harsh to force people not to wear modern helmets, even if it also takes away from the atmosphere.
A couple more thoughts:
First, here in Belgium there is a certain de facto distinction between 'historic' events like the RRVV on the one hand and 'retro' events like the blauwvelokeskoers on the other. The former are attempting to really create a historic atmosphere, and would obviously be spoiled if everyone just turned up wearing whatever. The latter are more just fun events for people who like to ride old bikes. Both events seem to coexist quite nicely, and there is room for everyone if they do so. I guess the organisers of any British equivalent would have to choose what they want to go for.
Second, none of the 'rules' mentioned have to be strictly enforceable, nobody has to be sent home for having the 'wrong' kit. But the organisers can make it clear that they strongly prefer it if people make an effort to fit in with the intentions of the event, and advertise the rules widely and clearly so everyone knows.