On the other hand we've been putting MCBs, RCBOs etc in outdoor boxes for decades in things like caravan hookup (and similar for boats) ...
Although, until quite recently, mostly plastic boxes. But I agree, no issues when not in direct direct line of pouring water or salt spray.

Mike

gkenyon said:

only sealed against water but not gas

Then I would say that the enclosure is not sealed.

Of course, any enclosure with a door is not sealed either when it is opened.

And about the only thing sealed by that definition, is a potted joint, lids with O rings gaskets and so on do not hold against pressure unless piston sealed or clamped to a reference profile at close intervals, and all glands and screw heads need dowty washers which on electrical stuff they never have...

In reality it always all leaks, slowly . And don't use stranded wire through compression glands and expect a seal either - air will pull along it between the strands once you get over a few inches water gauge of pressure, and thermal variations over a day/night in the UK for a truly closed  box in the sun can easily be a few hundred cm of water gauge  (a few PSI) between inside and outside.
Mike

Then, I think that we agree that ordinary electrical gear is not sealed in the precise sense of the meaning.

Even submarines are not necessarily as sealed as you might think. Water can collect in the bilges and be pumped out as with any other ship.

I have a number of MK switches in my outbuildings at the bottom of (galvanized) conduit drops. All have a hole in the bottom corner, but they also all contain a few corpses of wee beasties.

This month’s issue of Wiring Matters discusses this topic. Regarding the use of utility fuses, highlights the risk of condensation in outdoor electrical enclosures. “Such condensation can be problematic as it may reduce the creepage and clearance distances of the devices and assembly, potentially posing a shock risk. If the protection for these external enclosures relies on the supplier’s overcurrent protective device, it’s crucial to note that this type of fuse offers minimal shock protection and is unlikely to react unless the supply conductors within the enclosure come into direct contact. Designers and installers should refer to the fundamental principles outlined in Chapter 13 of BS 7671:2018+A2:2022”.

Rightly or wrongly I have installed quite a few external enclosures for EV chargers. I accept that there is a potential risk of moisture causing problems if they are opened in the rain. But from my experience at least they don't seam to suffer from moisture build up on a day to day basis, certainly over the last two years at least for the ones I have installed. And I have opened older units without evidence of issues.

I have a theory of why.
All the photos of units with problems shown so far don't have doors that can be opened to expose the front of the protective devices. I believe that the seals for these doors are under less pressure than the main seals and cable gland,  therefore end up being the path for entry and exit of air. These seals are quite well protected from moisture by the doors, therefore there is little moisture to be drawn in.

In addition to this most charge points are used at least once a week for a number of hours, with presumably the protective devices dissipating 10 to 15w of heat, would this be enough to get rid of any moisture that did get in from damp air.

I wonder how the risks from damp in an external CU compare to the risks involved in drilling through walls to get an 18mm cable in to the back of a consumer unit, along with all the cable disturbance. I can't believe that many if any EV installers would be testing every other circuit they may have disturbed.

There is an option of a second DB inside. OK in a garage or under the stairs but far from popular if the CU is in the hall of a nice relatively new house or similar.

AMK said:

May I add that upon inquiry, it was found that none of the usual wholesalers were familiar with or had stocked this particular plug.

 I think it's probably because they've not been asked for one

It helps if the enclosure is "stood" off from the wall: The wall will always be at a different temperature to the enclosure and so cause problems,

Note BT telephone distribution boxes which have a stand off mounting for this reason.

Years ago we researched cabinet faults which immediately cleared when the linesman opened the cabined doors.

A "ball" of moisture wandered around the cabinet faulting  different circuit terminals as it went around.