If the R0 number is somewhere around 3 (and I think this is perhaps a rather conservative estimate), then you should expect the R number to approach near R0 again if we lift all the lockdown restrictions. If R is around 3 with nobody vaccinated, then the virus will grow if fewer than 2 in 3 people have antibodies (something like 90%+ of those who are vaccinated and an unknown proportion of those who have had the disease). If we declare the high risk demographic safe because they have all been vaccinated, and therefore we can go back to licking each others' eyeballs, then you would expect the entire population to become infected in a matter of a few months.

0.3% of the entire population is more people than have died in the UK so far from the virus.

But more importantly, a much greater proportion of people will require medical help, and then survive, when infected. If you allow the virus to run rampant, then you will end up with the health service overwhelmed with people who need help. Those poor health service workers need a break. If we overwhelmed them, then we might discover just what proportion of infected people die without medical intervention.

To stop the virus growing in the population, we need at least 2 in 3 people to have antibodies. In order for that to happen, we need to have about 75% of people vaccinated, in order to account for the small proportion of people who won't develop effective antibodies after vaccination. We can only lift all the restrictions once that has happened. That is the point at which the herd immunity prevents the virus from growing, and protects the unvaccinated.