I find it very weird that £50 notes are so scarce, €50 notes are bog standard here and would have thought £50 notes would be common enough in England. Guess it's because you're not ripped off to the same extent as us everywhere you go
With regards to the op, if a shop refused to take my money I'd leave whatever goods I'd gathered at the cash desk, walk away and spend it elsewhere. Money is money regardless of what form it takes (so long as it's legal tender for the respective country).
I do hate it though when people buy tickets at the box office with €100 notes though, cleans out the float instantly first thing in the morning