All of this conjecture misses some important points.

1. Vaccination does not select for virus variants. It just is not effective against variants that differ substantially from the vaccine construct.
2. Coronavirus mutations occur in the infected host, by enzymes that alter the viral RNA. This is actually a defense mechanism, although it can create an undesirable mutation.
3. One shot of a mRNA or recombinant vaccine (Oxford/Astra, J and J), can actually induce enough cell mediated immunity to be protective. That seems to be the bet in the UK. It has a reasonable scientific basis, but it’s still a bet. Nature always has the last word.