You're still concentrating on the wrong thing. If you've come up with Slovakian/Spanish chalk streams in no time flat, then the combined forces of The Guardian, Wiki and the WWF probably could have done so too. Assuming these august bodies wouldn't lie for no reason about something so easily falsifiable we are left with the following two possibilities
1. They are speaking the truth and what you have come up with are not "chalk streams". Since they clearly are chalk streams to some extent, that will provide us with exactly what we are looking for -- ie the difference between the classical Hampshire chalk stream and these other lesser breeds you have erroneously but helpfully identified.
2. The Big Three are wrong. The reason(s) they are wrong, since we now know it won't be for any of the perfidious reasons you advanced earlier, will again be just what we are looking for. Something that leads to tunnel vision and category errors. Now the decks are cleared, we can find out what they are
River Unica is a unique river, which is well known all around the world by its fascinating beauty, amazing mayfly hatches, big graylings and brown trouts. Legend of fly-fishing Edgar Pitzenbauer said it was one of his favourite rivers. Unica is a typical chalk stream which runs through unspoiled nature offering a great scenery and fly fishing.
So, another
unique river (and therefore the 85% British figure would appear to be intact). Find out what the unique bit is all about and we should be in business. But please, Borry, try to wriggle off the orthodox hook for a little while.