I'm not in the UK, but surely you guys have some sort of equivalent to Gumtree/Facebook Marketplace, as well as semi-legitimate cash-only businesses and tradies that are trying to avoid tax?
Gumtree/Facebook Marketplace is still mostly cash in the UK - neither the buyer nor seller really trust the other with their bank details. Nobody is 100% sure that bank transfers are irreversible (they are, but if you complain loudly enough to the sellers bank they might mark the seller as a fraudster, ban them from banking for life, and transfer the money back to you).
Cash protects both buyers and sellers more from crazy people.
Nothing to do with cashless, most places don't accept £50 notes and cash machines often only dispense £20 (apart from airports). Using less cash will probably decrease the use further but so far the lack of £50 note usage had little to do with card payments.