The vast majority of the evidence shows that social distancing (alone) reduces the transmission rate of Covid-19 by around 75%.
The evidence also seems to show that when social distancing is NOT maintained (possibly when it's impossible to do so, as on a crowder Tube train), wearing a face covering reduces the transmission rate of Covid-19 by no more than 15% at the very most. Wearing a face covering when social distancing IS maintained seems to add VERY little, if anything at all, to a reduction in the transmission rate of Covid-19. (It might also lead some people into a false sense of security, making them think that they need to attribute less importance to maintaining social distancing).
So there was no reason to suggest that wearing face coverings would help in the reduction in the transmission of Covid-19 while social distancing could, and should, have been maintained. It's only now (when some people will find that they can't remain two metres away from others, as when they have to use publuic transport to return to work) that it makes any sense to advise the wearing of face coverings.
Further the new advice does NOT refer to 'face masks' (as mentioned in your post), TC44. It refers only to 'face coverings'. The official advice (both from the WHO and from the Government) remains that, to avoid shortages of PPE occurring for key workers, purpose-made face masks should ONLY be worn by those who're in contact with people who're exhibiting the symptoms of Covid-19 and by certain other health workers. Everyone else should be using a suitable 'face covering', such as a scarf, when travelling on public transport.