What it becomes is the electrician's design and they are then responsible for meeting all of the relevant safety directives and documentation responsibilities. That is a massively troublesome task so the simplest option is to install a CU that is all from one manufacturer's list of compatible products (i.e. that they certify as type-tested).
As for those CUs that have been repaired by others using different MCBs, etc, then the BPG#4 guidance is the sensible approach to determining the risks:
[
C3 code for] Mixed branded switchgear components within a consumer unit or distribution board where:
- there are no signs of thermal damage to component or associated connections
- the enclosure/assembly hasn’t been modified to allow installation of the component
- the component is securely fitted and all connections are adequate
- correct manual operation of the component
- direction of use of toggles/switches is the same as existing devices
Note:
If any of the above criteria is not met, this would warrant a code C2 classification to be recorded.
Points 1,3, and 4 in the above list are essentially saying the parts themselves (and related connections) are sound and functional. If you had same-brand parts you would expect exactly the same aspects to be met.
Point 2 is the one that prohibits bending/modifying of busbars to forcibly fit them to the devices (along with point 3) as well as prohibiting modifications of the enclosure. That is about maintaining the essential aspects of protection and current carrying (normal and fault) that would likely apply to the parts originally type-tested.
Point 5 is a basic safety aspect that the folk in the home, as well as some electricians, would assume operations are consistent and any inconsistency brought about by part substitution introduces a real additional risk for safe isolation.
Of course you see some cases where this looks odd (as someone pointed out the plug-in Wylex breakers operate in the opposite direction to the old rewirable Wylex CU main switch, but at least all plug-ins operate the same way, and the manufacturer is offering them as approved for use) and you get some SPD that are "universal fit", but they are not normally going on the busbar but a cable to main switch or via an approved MCB, etc.