But I agree this is likely unmeasurable.

By the way, being hard to measure doesn't mean it doesn't exist. And there are plenty of things easy to measure that are meaningless

>> being hard to measure doesn't mean it doesn't exist

No in this context it does. Either you can measure the increase purchasing power or you can’t. And if you can’t measure it, it doesn’t exist in this specific case.

The difference is that when BoE destroys notes they are accounted for by either the issuing of new notes and or crediting their withdrawal with a bank-entry credit thus the money in circulation does not change.

Burning notes willy nilly effectively reduces the amount of currency in circulation because the central bank thinks the money still in circulation and doesn't replace it. This is why in many countries defacing or destroying the currency is unlawful.

Clearly, in modern times in modern democracies it's very unlikely that in any practical scenario that some entity would be capable of unofficially adding or withdrawing enough money from a currency to actually destabilize it, moreover in such an event the central bank/government would be onto it in seconds.

It's however an altogether different scenario if the citizenry somehow has the perception that the currency isn't exactly what the central bank purports it to be. Just the notion that something may be wrong with the currency could destabilize it, hence hellishly tight laws to ensure this never happens.

(Miniscule until I manage to stuff my face with enough donuts.)