I received the Equiniti request, as well as a similar one from Civil Service Pensions. Asking for the same details but with different qualifications for the witnesses.
One allowed the Bank to witness it, the other did not but required, amongst others, Officers of the Active, Reserve or Retired list but only if they had known you for at least a year.
My bank also required a similar form but without the need for witnesses.
It appears to be a Tax requirement for those of us who have to live overseas and are forced to drink copious quantities of the reasonably priced local vino. Very tough down here in the Bordeaux region, but,hell somebody has to do it.