What water temperatures are being recorded around the country on your rivers that are helping to make the decision to not fish? Air temperature is, as in winter is not always in step. The D.Stour and H.Avon which run so close will be completely different at present. Saying its irresponsible to fish if the air temperature is over 30c is rather meaningless.
That said, water temperature is only one factor. Oxygen levels in two bodies of water of equal temperature could be vastly different. If I took two buckets of water, one still the other with an air stone the dissolved oxygen would be different. To the point that you can manipulate a bucket of waters pH by doing this. A polluted slow flowing body of water would be affected disproportionately to a clean one of a higher temperature.
What I’m getting at is that not every river is going to be the same and when a spate river in the midlands is not suitable to be fished a southern chalk stream may be completely different. I recall a big river I swam in, in southern France on a very hot trip, it was FREEZING because it was mostly ice melt from the mountains.