MIG wrote:

Deleting anything from databases always seems risky practice to me,
but I suppose it would be archived somehow.

True but in this case they have multiple counties in the database to handle
the various changes over the years (and even list some unitary authorities
as "counties") and it's probable that the regular shake-ups of local
government are creaing so many variants that it's getting too much for the
system. Having to field endless complaints about people being in the "wrong"
county can't help either.