I've mentioned on here before I have a friend who tests river water quality for several suppliers in the South West and Midlands regions (so your Wessex, Thames, Southern etc), and between early-mid 00s up until very recently (say 2016/17) water quality in all of those areas was, for the mostpart, better than it had been for decades previous, and there was a huge increase in the variety of water based and bankside vegetation as a result, along with a general increase in the associated wildlife. In the last couple of years (coincidence or not, since water and sewerage were deregulated - anything familiar there? cough-gasandelectric-cough), however, there has been a massive increase in major pollution incidents all over the country and water quality has dropped off a cliff - my friend said the shortest and sharpest decline they've seen since records from the second world war, at least in terms of general nationwide water quality.
I don't believe for one second that bad water quality is good for fish or fishing, and the barbel decline is, I think, a completely separate issue. The former can, I think, be simply explained by the massive drop in number of people fishing the rivers, with far less bait going in on a much reduced regularity, so of course 100lb match weights aren't going to be a thing (at least in rivers) now. Otherwise one cannot argue with the fact that silvers and eels have made a huge comeback in many rivers over the last decade or so, and we all know that lots of silvers and eels = good water quality and a healthy and balanced ecological system. Ditto grayling making a comeback in many rivers as well, as Lawrence mentioned above.
As for the barbel - I've said before I don't believe otters are solely to blame for the decline. I think just as much blame can be apportioned to almost annual flash floods over the last 10+ years, subsequent gaps in year class, and a huge drop-off in the amount of barbel fry being stocked (go back to the 80s and 90s and the EA were stocking many of the main rivers on a fairly regular basis. These days your Hampshire Avons, Wyes and Severns might get a few hundred fry every two or three years?), not to mention, again, the lower number of people fishing for them, so they simply don't get caught as much.
I think (the royal) you need to be careful about dismissing this new wave of pollution as a minor concern, even worse marketing it as a "good thing" - lest we forget there's a lot more going on in and around our rivers than just our fishing...