That is almost certainly wrong. Vaccine uptake in under-12 is 0% and vaccine uptake in young adults is low. There's no way to make those kinds of seroprevalence numbers work.
The big caveat is that the sample population consists of blood donors, which may or may not be representative of the overall population.
And yes blood donors aren't representative of the general population due to selection effects.
The result of the report is that some form of immunity exists in 97%+ of the blood donors at least 17 years old, in the UK.
The most you receive is a cup of coffee and a biscuit.
But then again, I seem to remember that the 97% number was not a fair enough random sampling.