I think it's more to prevent you from sitting on a mountain of cash the government is unaware of. Generational money, laundering and the like. Forces you to admit to all your cash or lose it. It smells pretty evil and totalitarian to me.
It's routine, it's happened to 13 notes since 1990: https://en.wikipedia.org/wiki/Bank_of_England_%C2%A320_note
However this has a lot more to do with the supply of oil and gas than the number of banknotes in circulation.