Your question does not have a clear-cut answer, because it depends on how you define and classify the MCB enclosure and its function in the electrical installation. I think this is a matter of professional judgement and risk assessment that should take into account various factors.

Without doubt, it is not a consumer unit.

I doubt that it is any more similar than the typical "REC2" isolator which one finds nowadays betwixt the meter and CU.

The difference between this MCB and just a plain isolator is that it contains current sensing which will produce substantially more heat and it needs to be able to operate under several kA fault conditions and quench high energy arcs etc.
Would a plastic (,albeit "flame retardant PC",) enclosure as pictured be deemed suitable?

And if so what difference would this be to say an RCBO in its own consumer unit to supply a shower?

Does the enclosure:switchgear comply with BS EN 61439-3 ?

No. For reference, the MCB (OEM, often rebranded) is a projoy PEBS-L and it has an associated enclosure for it:
https://www.projoy-electric.com/Products/Circuit-Breaker/PEBS-L-DC-Miniature-Circuit-Breaker/PEBS-L-Series-160V-63A
https://www.projoy-electric.com/Products/Circuit-Breaker/PEBS-L-DC-Miniature-Circuit-Breaker/Weatherproof-Enclosure

Some people have been using 4 module metal clad consumer units instead from other companies.

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.

For installations of energy storage systems in dwellings, I would recommend that a set of monoblocs or cells is housed in a non-combustible (metal) enclosure which also contains the overcurrent protection for that battery array.

That may well mean you you may not need to worry about the plastic enclosure that the overcurrent protection is housed in, see Regulation 421.1.201 (ii).

Akbar Ramzan said:

Many installers are using plastic enclosures however to house a double pole MCB which acts as a means of isolation as well as overload and short circuit protection. This seems wrong, especially when dealing with a DC source with high energy withstand requirements. They are using this in escape routes within the house.

The batteries should not be in escape routes, to be honest. Consideration of not impeding escape routes (that would include in case the battery has a fire or fault) is in IET Code of Practice for Electrical Energy Storage Systems, 2nd Edition, Section 11.2.3, Table 11.1.

Ok, yes that is ideal but what about the majority of existing batteries which require external overcurrent protection as per manufacturers instructions?

Akbar Ramzan said:

Ok, yes that is ideal but what about the majority of existing batteries which require external overcurrent protection as per manufacturers instructions?

Are they "batteries" or "monoblocs" that are strung and paralleled together? Usually, I think, the latter.

Monoblocs can have high prospective short circuit currents without overcurrent protection. This provides risks of arcing and fire. Limiting any unprotected cables to be within the metal enclosure containing both the batteries and overcurrent protection reduces that risk.

Domestic properties are not really suited to the "battery room" or "battery container" that might be found in commercial and industrial battery installations, simply because the premises is under the control of ordinary persons and children (whereas in a workplace, risk assessments lead to measures being put in place to prevent access except when supervised by skilled or instructed persons).

Some monoblocs are currently installed by just putting them on the floor, so they are susceptible to damage (accidental or otherwise), or if they fall over can pull cables out etc. ... very poor practices. In addition, we need to make sure that ordinary persons and especially children can't get at the "click to connect, push button to remove" type connectors that can be removed (on load) without the use of a tool.

All, of course, preventable by a more sensible installation design.

Manufacturers of certain monoblocs provide suitable metal racking solutions, and the overcurrent protection could be installed in the racking solution. This is a strong recommendation I would make.

I do understand that the designer needs to think about temperature, but it doesn't make things impossible - other manufacturers provide complete solutions already in a metal enclosures with integral overcurrent protection.

PAS 63100 (see here: https://standardsdevelopment.bsigroup.com/projects/2022-00181#/section ) will hopefully be available very soon from BSI, and provide some clear requirements that I hope will address issues related to fire safety and domestic storage battery installations. Just to caution, that the final version of standards is rarely the same as the draft that was put out for public comment, as they are updated in response to comments received.