If you look at RHS plant-finder, under Rheum x hybridum "Champagne" you will find the wording "tentatively accepted name", which I suppose means that it isn't a well-defined variety. Two possibilities occur to me

(1) it is Stein's Champagne, an established variety with AGM, but very rarely available.

(2) Champagne rhubarb was used as a synonym for forced rhubarb. Then people turned up wanting to buy it as if it was a variety, and some unscrupulous seller offered them whatever they had to hand and called it Champagne. Informal networks then spread it out to people's gardens. (With easy-to-propagate plants, this is a remarkably effective method - a rare Chinese plant previously unknown to science or nurseries was discovered about 10 years ago as a houseplant common in Scandinavia, and traced back to a missionary who brought it back early in the 20th century as a memento of his stay. Likewise, the prolifically self-seeding giant Echium (E. pinninana) is rapidly becoming very common in England without it being easily available in garden centres as more of us get it to over-winter.)

My father claims to have "Champagne rhubarb" in his garden. It is more tender to the bite than my Timperleys, but so it ought to be growing it on moist Somerset clay as opposed to my dry stony soil. I ought to take a division and see if I can spot any difference.