The regs are more general, it is really a reference to any assembly of electrical parts. In the domestic world that really comes down to the CU, but beyond that it really applies to anything to put together to do a job in an installation.
Some aspects of the standards cited are only applicable to big systems (forget the details but over 100A at least) where it covers aspects like would busbars bend under huge fault currents, etc, etc. But generally the way it is interpreted is as
@timhoward say - stick to a supplier's pre-assembled CU or at least the list of parts they declare as compatible.
While I am normally happy with sane and reasoned attempts to replace difficult parts instead of ripping the whole lot out, when presented with something like that I would agree with
@westward10 and feel very uneasy at re-using something obviously bodged and of unknown provenance. Especailly as the cost of fixing it properly would be dominated by the skilled time, not by the likes of a budget Fusebox (or similar) CU where it is known to be as specified..