The PHE report measures both antibodies from natural immunity, and antibodies from (vaccine or natural infection or both). (They don't have a vaccine-only measure.) The former is in the 20% range, depending on age. The latter is in the 97%+ range, again depending on age, reaching essentially 100% for older people.

The big caveat is that the sample population consists of blood donors, which may or may not be representative of the overall population.

Are there any under-12 kids in the blood donors?

And yes blood donors aren't representative of the general population due to selection effects.

You can read the report, but the blood donor population is 17 years or older.

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.

And blood donors are either socially responsible (probably vaccinated) or economically disadvantaged (probably exposed).

There is no economic incentive to donate blood in England (probably UK/GB either).

The most you receive is a cup of coffee and a biscuit.

Natural immunity?

But then again, I seem to remember that the 97% number was not a fair enough random sampling.