As a matter of pedantry, there is still no definitive evidence that mask-wearing is effective in its own, so it's not a matter of being "right" or not; nor is it fair to "blame" the scientists for dithering, since, after all, it isn't their job to make decisions but to gather and interpret evidence. Evidence on the effectiveness of masks is always likely to be subject to various caveats, eg you need to take into account other measures or people not wearing them correctly, etc.
Likewise, I doubt that the Scientists on SAGE have made up their minds suddenly since Sunday morning, when Government advice was still to wear masks (or equivalent face-covering) but not to make it mandatory. This is a political decision, not a scientific one.
I have to say I am surprised that ymb is calling for Johnson to go over this, or viewing it as the last straw perhaps. Surely, it's worth enduring for a while longer considering his role in Brexit and "the Swamp"?