The standard states they are only suitable for use where the circuit already has RCD protection.are they explicitly forbidden?
That makes them useless.
They could still be used, but the reason for using them is then undefined in BS7288, and not recognised by BS7671.
Compliance with BS7671 and any other standard isn't mandatory, but is generally accepted to be a suitable method of ensuring safety.
Not complying with BS7671 means it can't be relied on as evidence of safety or compliance with Part P or the EAWR.