How are you going to conduct randomised control trials? Why do you need to? If the virus is carried in saliva droplets as you exhale, then any measure which reduces droplet emission will obviously reduce the chance of you passing any virus to someone else.
It really is that simple. Sadly the devolved administrations have used it as an opportunity to play politics and show their differences with HMG.
Here's an RCT
Doesn't give you the answer you want to hear though so let's ignore it.
Why do an RCT? Because mask policies come with all sorts of costs and if they are shown not to be effective then those costs are not offset by any benefits.
One of the most important costs is the resentment and loss of trust in Government regulations and laws at a time when said Government is really keen to get everyone on board the vaccine bandwagon.
Nearly a year of lies, spin and misrepresentation of data do not endear people to believe a desperate Government begging people to take the vaccine it approves.
"It really is safe! Look, we got Vallance to do a graph showing you how safe it is!
LOOK AT THE GRAPH OF SAFETY! LOOK AT IT!"
The death of trust in Government and connected institutions. The price of policies like this:
“A substantial number of people still do not feel sufficiently personally threatened,” they write. “The perceived level of personal threat needs to be increased among those who are complacent, using hard-hitting emotional messaging.”