Whilst BS7671 is not law, it is not even best practice.
It is the bare minimum considered acceptable.
What is wrong here is the "council bod" citing compliance with BS7671 as the reason for wanting this unit changed out.
Best practice could certainly be considered as utilising the CU manufacturers' SPD.
It could also be a policy that they have.
In which case, why not state this?
To state that the use of an SPD by a different manufacturer to the CU, which is otherwise of all the same make device, as a non-compliance with BS7671 is incorrect.
However, it is true that the regs do say that for a domestic installation, we must use a type tested CU. If we include anything in that CU that is not part of the manufacturer's type testing/acceptance, then we have invalidated it. So technically, putting anything other than the manufacturer's own bits (or bits they specifically approve) into it means we don't comply with that particular rule.
An SPD does not form part of the type testing referenced in BS7671.
The only reason this clause has been included in BS7671 is that electricians were ignoring the requirements of the legislation and the other relevant standards with which they should have been complying.
The requirement for using all the same make devices has been in place in the relevant applicable standard since 1991, in round figures 32 years.
For most of those 32 years, sparks ignored the requirements.
You could ask, did they need to comply with this standard for all those 32 years?
The standard was listed in BS7671 Appendix 1, but it was not made explicitly clear if compliance with the standards listed in Appendix 1 was compulsory for compliance with BS7671.
If we look back, as of 2008, Appendix 1 in BS7671, is listed as Normative.
Therefore, compliance with the standards listed therein is compulsory for compliance with BS7671.
In 2012, the product standard for consumer units for domestic premises was revised, along with the remainder of the series being revised between 2011 & 2013.
The relevant parts were revised and published in 2011.
So, if we are generous, the legislation and relevant compulsory standards have been in place for 12 years (in round figures).
These “relevant” standards were the BS EN (IEC) 60439 series, updated in 2011-2013 to the BS EN (IEC) 61439 series.
The specific parts are art 1 & part 3.
It is part 3 that requires type testing due to a country-specific requirement for the UK in Annexe ZB of the standard.
This relates to the breaking capacity of the assembly.
This breaking capacity is related only to the circuit protective devices, i.e., the "circuit breakers”.
An SPD forms no part of the tests for the breaking capacity of the assembly and, thus, no part in the type testing.
Therefore, any SPD can be used in any board without impacting the type testing.
The caveat is that if an MCB is used to protect the SPD, then this MCB will form part of the type testing.
The type testing is limited to the breaking capacity, thus, the interrupt rating of the assembly.
The product standards are much more closely linked to the legislation than BS 7671 is to EAWR.
The legislation is the Low Voltage Directive (LVD), integrated into UK law as the Electrical Equipment (Safety) Regulations (EESR).
Within the legislation, statements are made that compliance with the Harmonised standards will result in compliance with the requirements of the law.
The same is now true with the Designated standards to UK law post BREXIT.
Harmonised documents are published in the Official Journal of the European Union (OJEU).
Designated standards are at least indicated on the British Standards Institute (BSI) website and published in official UK post-BREXIT legislative documents.
The safety requirements of the legislation, LVD/EESR, are known as the Essential Health and Safety requirements (EHSRs).
It is these EHSRs that must be complied with.
They are very broad in the LVD/EESR, so the Harmonised/Designated standards are utilised to establish the details of the product requirements for compliance with these EHSRs.
I hope this illustrates that the standards and legislation are intimately linked in documents that will stand in court as valid and legitimate means of compliance backed by published legislation.
These harmonised and designated standards contain references to how they are used for compliance with the law.
The law contains references on how to use the standards to comply with the ERHSR requirements.
So, in summary:
Any SPD is fine.
The requirement doesn’t come from BS7671.
The requirement for type testing is from the product standard and is considered a requirement for legal compliance.
An SPD does not form part of the type testing requirement.
I hope this helps.
The "Council bod" can refuse the off-brand SPD, but to state a non-compliance with BS7671 as the reason for this is incorrect.
It may well be in the OPs' best interests to replace the off-brand unit with a matching one to save the hassle.
The "Council Bod" needs to be educated to ensure they act on the correct information.
From the OP, the "Council Bod" does not appear to be making decisions based on the facts related to the scenario.