Thanks for all the replies. It was the CIFAS referral I was thinking of, but I have not been the victim of identity fraud, and hence can't get one. I never buy things on credit apart from normal use of a credit card, paid in full each month. So, I thought I would only gain by putting such a marker on my record. But, if it can't be done, it can't be done.
BNPL offers aren't of interest to me. I believe that the fundamental theory of BNPL is that consumers will forget about the lump sum to be paid later and pay a whole lot of interest on the full sum. I think that would be a realistic model of my likely behaviour.
The service offered by Experian is sort of interesting, but not quite enough for me to go for it. It would be cheaper for me to order my credit report every three weeks or so.
Cheers,
Ross-c