Might work in Aus but the UK police seem to have been highly reluctant to enforce any of the coronavirus social distancing provisions..cf. multiple stories of quarantine blatantly being breeched, mass gatherings..as a result I think the chances of getting the boys in blue to attended a flight arriving at LHR because Mr Bloggs in 13A wasn't wearing a mask would be slim.
And there is the underlying problem, as Dave Reid UK has pointed out - what is the legal situation on the aircraft?
The UK Gov has been very keen on issuing
guidance on the precautions to be taken regarding coronavirus but very wary of codifying such guidance into law, even if it is codified there seems to be a whole host of allowable medical reasons for not wearing a mask and no requirement for the non wearee
to be able to provide supporting documentation....
This from
Gov UK, face coverings and exemptions..
Those who have an age, health or disability reason for not wearing a face covering should not be routinely asked to give any written evidence of this, this includes exemption cards. No person needs to seek advice or request a letter from a medical professional about their reason for not wearing a face covering.
and then add into that quagmire the issue of would the cabin crew insisting on a mask being worn be a "lawful command".