I've updated British Airways - Although I couldn't find a URL as comprehensive as LH!
There is no guidance as to what a Facemask is (e.g it doesn't specify it must be PPE), but having flown with BA a few times recently it can be anything that covers your mouth/nose (e.g a scarf). Facemasks are not provided, although if you break your mask you can get a replacement. Free facemasks are available at London Heathrow (at least they were on 20th August). A Face shield does not substitute a mask, but you can wear a shield over a mask.