Institutions are at liberty to make up their own in house rules.
Interestingly the 'certify by donor with capacity' is not included in primary legislation
Section 3 in https://www.legislation.gov.uk/ukpga/1971/27
It's a COP add on
Does the following section in effect mean the same as the OPG guidelines with the exception that 'with capacity' is not mentioned?
3 Proof of instruments creating powers of attorney.
(1)The contents of an instrument creating a power of attorney may be proved by means of a copy which—
(a)is a reproduction of the original made with a photographic or other device for reproducing documents in facsimile; and
(b)contains the following certificate or certificates signed by the donor of the power or by a solicitor [F4, authorised person ] or stockbroker, that is to say—
(i)a certificate at the end to the effect that the copy is a true and complete copy of the original; and
(ii)if the original consists of two or more pages, a certificate at the end of each page of the copy to the effect that it is a true and complete copy of the corresponding page of the original.