From what I recall from Dr. John Campbell’s excellent daily videos, the UK strategy was not to give only one dose, but to simply delay the second dose a few weeks so that more people could get their first injection from the initial supply(presumably supply will increase over time). That sounds much lower risk. I think the single dose proposal was a US thing.

All of the mRNA vaccines target the spike protein. Coronavirus spike protein mutations have only ever been observed to occur in bats- never humans. Therefore the number of vaccinated people will almost assuredly not increase the likelihood of a resistant strain.

I would accept a similar kind of reasoning in an argument towards caution, but not in an argument for seeing what happens with prolonged partial but insufficient immunity. Aside from near certainty that a change to the target protein is game over, we have no idea what other mutations could render immune responses triggered by the vaccines worthless, and I don't see the ethics in an experiment to find these mutations.