Re. the question of whether a permit system is helpful in conservation issues, consider the following:
Giants Hole: delicate formations out of the normal through route deliberately smashed and large sections of the broken formations removed from the cave (collected for sale?). Access control: the landowner insists that an access fee be paid by anyone who goes down.
Swildons Hole: Barnes Loop, when I first saw it in 1960 was magnificent but since then it's been battered almost beyond all recognition. Access Control: the landowner insists that an access fee be paid by anyone who goes down.
Ease Gill: Easter Grotto had gradually been spoilt over the years and now has broken formations and people have left, at various times, sweet papers, flashbulbs and the like. Access Control: permit from CNCC - no payment required.
OFD II: Trident: despite the area below it being taped off, the tip of the formation has been broken off and has had to be stuck back on. Access Control: permit required and also limit on party size.
OFD I: The Antlers: a number of years ago now the delicate helictite formations high up on the wall of a passage were broken off completely: Access Control: permit and (until very recently) leadership system and limited number in the party.
Water Icicle Extensions: No damage to the delicate floor deposits and all visitors keep to the taped route. Access Control: New passage gated and a leadership system, restricted dates and limited number of visitors agreed between DCA, the club and Natural England.
So you can see that a permit system doesn't necessarily protect the cave, though it may have some effect on visitor numbers. Ultimately, one careless person can start the process of deterioration; one person intent on doing deliberate damage and/or stealing formation for their own ends can irretrievably damage an area.
As someone said earlier: EDUCATION, EDUCATION, EDUCATION is an essential part of the process.
However, if necessary, couple that with the very occasional set of very specific restrictions if it is absolutely essential to protect something of unique value. But be very sure you before you go down this route that you have the full understanding and agreement of the local cavers since you will be relying on them to help you protect the cave.
Both DCA and CNCC have a great record of working with Natural England in monitoring caves and both have their own cave monitoring websites so everyone can see what is going on. A statutory duty of EN is to monitor SSSIs in their care and they rely on the local cavers do this work for them. This puts responsible cavers, working within the club/regional council/BCA system in a good light with EN - surely a plus if we engage in sensible debate with them about widening the CRoW definition to ensure that it includes cavers.