From note 2 below 536.4.203. Sounds very onerous and fraught with possible kickback from future inspections. I think most sparks here would just fit it blissfully unaware of the consequences and not worry one bit about it. Likely where Ipf is well below device ratings, no real concern about safety exists. However, with +45 years of experience under my belt, I have learned not to be as cavalier as I might like to be. 

So for me its a separate enclosure. I take your point about the resulting piece-meal appearance.

From note 2 below 536.4.203. Sounds very onerous and fraught with possible kickback from future inspections. I think most sparks here would just fit it blissfully unaware of the consequences and not worry one bit about it. Likely where Ipf is well below device ratings, no real concern about safety exists. However, with +45 years of experience under my belt, I have learned not to be as cavalier as I might like to be. 

So for me its a separate enclosure. I take your point about the resulting piece-meal appearance.

note 2 below 536.4.203. Sounds very onerous and fraught with possible kickback from future inspections.

True, but there is also the possibility that the future inspector is not aware of a carefully thought out  decision to adopt the responsibility.  In my occasional role as the design authority for making bits of kit, which among other things have other companies circuit breakers and so on inside them. Providing the evidence to declare these to be UKCA compliant, I often have the need to consider mix and match from different makers. almost always this can be guaranteed to be OK by design, especially by knowing that there is a suitable energy limiting fuse or similar upstream or from the construction of the enclosure.
note that many parts of the world that nominally follow the same rules as those from which BS7671 are derived, expect electricians to assemble the equivalent of the DB onsite and parts from more than one maker would not raise an eyebrow.

Mike.