Well what makes an MCB in a box into a consumer unit for the purposes of this regulation?

A tricky one. The risk to be mitigated is that some CU makes use plastics for the body that burn rather too well for comfort, This would be fine - there are lots of plastic parts an a modern house, but  some wiring  inside CUs have been linked to fires.

Now if is unclear how much of that is due to any one factor but there are several candidates.

1) the fact there are a great many wires crammed into one box and in some cases wires pulled violin string tight after a CU change, or

2) that some terminal designs on things like neutral bars that are supposed to carry up to 100A are remarkably lightweight single screw fixings

3) The available current at the origin is quite capable of not just overheating but  fusing the metal of smallest sized outbound cable. (assuming a 100A fuse and a 1mm lighting radial) So the MCB has to work for its living as well

4) terminals that accommodate a very wide range of wire diameter are necessarily a compromise.

What follows is a personal opinion. How similar it is, depends on how similar or different those factors are.

A simple arrangement like a REC 2 or a double pole RCD is very low risk - cable routing in at one end and out at the other means the cables are well seated into terminals that are designed for the large cores and wires are not unzipped a to reach destinations that in opposing directions and there is less risk  of  pulling at an angle or kinking n a way that may give a false tightening, There are also of course far fewer ends to terminate, and no high current neutral bus bar.

But, if you start to add things like multiple MCBs, toothed bus bars and terminals to split the neutral, than all these risks re-appear and once again it is more CU -like.

So to me how similar to a potential  fire hazard CU depends on how full it is, and how circumlocutory, and also how strained, the wiring looks,

Mike.

PS you may wonder why this is a UK only reg, and if other countries do not see the same severity of problem, that is another interesting question, but not an answer to the immediate question.

I agree partly with what you're saying but the definition of consumer unit and distribution board in BS7671 includes single circuits.
1) With batteries and cheap night rate tariffs, you could have continuous high currents for 4 to 6 hours for battery charging. (Octopus Go, Octopus intelligent)
2) True, the definition of distribution board mentions terminals for neutral and CPC
3) The fault current of batteries can also melt even 25mm2 conductors, the fault current is in the several kA range, especially with large capacity batteries or parallel batteries. The MCBs are typically 10kA rated. The BMS often relies internally on just semiconductors such as mosfets for protection which can fail short. Then the sole protection for short circuit is on any external MCB/ fuse.
4) You're often dealing with class 5 or 6 stranded battery cables which have their own issues

But the biggest difference between an MCB and an ordinary isolator is that it's going to produce more heat and it needs to be able to quench large arcs. I think these MCBs are even designed to vent arcs out a particular vent slot if it fails to be quenched.

I agree partly with what you're saying but the definition of consumer unit and distribution board in BS7671 includes single circuits.
1) With batteries and cheap night rate tariffs, you could have continuous high currents for 4 to 6 hours for battery charging. (Octopus Go, Octopus intelligent)
2) True, the definition of distribution board mentions terminals for neutral and CPC
3) The fault current of batteries can also melt even 25mm2 conductors, the fault current is in the several kA range, especially with large capacity batteries or parallel batteries. The MCBs are typically 10kA rated. The BMS often relies internally on just semiconductors such as mosfets for protection which can fail short. Then the sole protection for short circuit is on any external MCB/ fuse.
4) You're often dealing with class 5 or 6 stranded battery cables which have their own issues

But the biggest difference between an MCB and an ordinary isolator is that it's going to produce more heat and it needs to be able to quench large arcs. I think these MCBs are even designed to vent arcs out a particular vent slot if it fails to be quenched.