The WHO state:They aren't, but they are a mitigation which allows 2m to be reduced to 1m+ (there are others too).
Unfortunately in the UK we tend to try to impose additional restrictions, in order to be seen to be going above and beyond any sensible measures done elsewhere.for areas of widespread transmission, with limited capacity for implementing control measures and especially in settings where physical distancing of at least 1 metre is not possible – such as on public transport, in shops or in other confined or crowded environments – WHO advises governments to encourage the general public to use non-medical fabric masks.