Those signs about not challenging someone not wearing a mask are all over the place in the UK, next to all the signs saying you should wear a face covering (they never say mask). I discovered my sun visor is an acceptable face covering in the UK, which was quite good so I could wear that in my (empty) train carriage for my 3 hour train journey. (Had it been a full or even partially full carriage, I would have worn a proper mask, but since there was nobody to protect I took advantage of the UK lunacy).