Interesting thread - skipped last few pages as anxious to post! I think chances of a massive Barbel from a small river are largely gone due to predation by our furry friends, and any hidden or lightly fished stretches that are likely to hold an undiscovered big fish are even more susceptible to Otter attention with little human activity to keep them away (not that that seems to have much effect anyway).
My bet would be on a big river - Trent, severn or Thames, or tributaries close to their mouth, where big, slower fish could survive Otter attention.
The days of outsize fish in little rivers are gone - easiest prey for an agile apex predator like an Otter. We didn't realise it, but we had a slightly artificial halycon age for the last decade where we could feed up big fish in focussed areas with no real natural check on their upper weight limit. Fish could still grow huge on big rivers but maybe through age and not by feeding on HNV baits.
Shame - I loved catching doubles galore under my feet on the Kennet ten years ago, but it was all probably a bit false. Next Barbel record (as with Tench, Chub, Bream etc. etc.) will fall to a Carp angler fishing a river.