That was my point - that BS7671 is of very little relevance to the matter under discussion.
The one grey area, at least in my mind, is equipment hard-wired into the installation, whether through an FCU or otherwise, since I think that some would say that such constitutes 'fixed wiring' and therefore comes within the scope of BS7671.
If you read the definitions of Electrical Installation and then Electrical Equipment in Part 2, and then Chapter 11 does the grey-ness go away?
Equipment is in scope, but the requirements of the regulations apply only to its selection and use.
HOWEVER...
I do not see why a manufacturer may not require things of the installation which do not contravene the regulations.
There is no argument which can ever be made which says that the Wiring Regulations forbid the use of an FCU with a 3A fuse to supply an item of equipment.
There is no argument which can ever be made which says that the Wiring Regulations (or indeed any others) forbid the use of a 3A fuse in the plug which an item of equipment.
Ib ≤
In ≤ Iz is
required.
Ib ≤
In << Iz is
not forbidden.
We don't know if they do, but it cannot be argued that gas regulations (or indeed any others)
may not require things of the installation which do not contravene the Wiring Regulations, such as a 3A fuse.
And I don't think that it can be argued that a maker
may not require a 3A fuse. No matter how much you say they should not, no matter how many times you ask them to justify the requirement, no matter how many times you ask them to explain how their equipment is used in places where such fusing cannot be provided, ATEOTD they may make that stipulation.
The Wiring Regulations require one to "take account" of the manufacturer's instructions, so it can be argued that any MI imposed requirement for a 3A fuse is not mandatory.
I leave it as an exercise for the reader to decide whether ignoring a manufacturer-required provision of a BS 7671 compliant safety device just because
you don't think they
ought to require it and/or just because
you don't think their equipment
needs it, qualifies as good workmanship and demonstrates an appropriate level of skill.