English and Scottish council tax bands were set in 1991 by valuers doing drive-by assessments at speed. A meaningful number of them are wrong, and the challenge process is free — but it requires evidence, and assembling that evidence by hand is tedious enough that most people do not bother.
So: pull comparable properties on the same street, find the ones in lower bands, compute the annual difference and how far back a rebate would run, and generate the appeal letter with the comparables cited.
It is an MVP. I have not turned it into anything anyone else can use.