As you know Alan I've fished that stretch for a good 20 years now and the rules there (which are admittedly many) have safeguarded the stocks of what has to be the most prolific small river in the country for barbel.
You also got to remember that other species are being fished for so there will be hooks left in fish, from those after chub, dace and roach, barbless do drop out and I can remember only catching one barbel with a barbless hook in it in all that time, so thats a ratio 1/5000. On the flipside there's been about 50-100 I've caught with a barbed hook left behind, maybe from club members breaking that rule or most likely from the free stretch upstream where the fish go sometimes. Sometimes these hooks staying in form a growth round them, old fashioned barbed hooks have no place in catch and release imo, micro barbed are far better, whether they would be better than barbless long term who knows, maybe if it ain't broke....
Any hook's tearing should also be negated by not fishing too heavy, it's not a hit and hold venue and pretty snag free anything over 6-8lb isn't really necessary, as you say the mouth damage is pretty limited aside from the odd fish, when you consider how heavily fished it is it's quite a credit to the anglers fishing it compared to some places.
Ps well done for getting in those swims so often, I't takes me ten years to get in those spots as much as you have in the last two weeks