they thought that masks protected them from others (rather than vice versa- which they do to a limited extent) AND that Covid-19 posed a personal risk?
Masks protect both the wearer and those they are around. In addition if everyone wears them then we are all protecting each other. There are probably a limited number of people who genuinely cannot wear masks for medical reasons but some countries have a very high compliance.
I think that if the government had said that masks protected people from the beginning rather than saying that they only protect other people AND if people realised that even 20/30 somethings can be very ill (not necessarily die in most cases, but definitely suffer a prolonged period of feeling really unwell) then more Brits would be willing to wear them.