It might be technically possible to run an EV charger from a distant secondary Consumer Unit, but it's bad practice.
There will be considerable voltage-drop along the cable between the two Units.

The EV charger is likely to be the device which draws the highest current, and will ideally have an MCB rated at 32A or above, and its own Type-B RCCD.

When you look at DBs you'll notice that electricians arrange the circuits with the high-current trips closest to the Main Switch,
and the low-power lighting circuits further away.

So you want the connection point for that EV charger as close as you can to the Main Switch, not hanging off a sub-box in the garage with a bit of string between them!

Although I could've shuffled my circuits around, I've got a separate steel Consumer Unit below the existing one. That houses the RCCD and MCB for the EV charge point.
There's a couple of 10mm² wires which connect the EV trips directly back to the output of the Main Switch above.

Connecting together multiple consumer units is permissible, provided that there are metal bolts holding them together,
and all cables between them pass through insulated 'shields' to prevent them being touched.

Consumer Unit manufacturers offer 'joining kits' which include the brass bolts, thick wires and an insulated shield.

The insulated shields available for my MK Sentry enclosures weren't quite what I wanted because the wall behind wasn't completely flat.
So I made my own with a 3D printer, incorporating a 10mm-wide groove.

Before connecting the enclosures together, that groove is filled with intumescent strip.
If there's a fire in one consumer unit, the strip rapidly expands with the heat, completely filling the cable-passage with 'foam'.
That prevents flame and smoke from passing between the consumer units.

We have two car chargers with different plugs🙄 - a 32A one and a 10A one.  The larger one is fed with 6mm2 cable from its own tiny CU (looks just like the pic that Transparent posted) which is fed from the main tails using henley blocks.  Don’t know why the sparky used Henley’s rather than coming from main CU, there was room. 

The smaller one is just a leaf granny cable that came with the car, plugs into a socket.  We used to have a ‘proper charger’ for it, and had intended to sell the leaf by now… but Mrs Robl doesn’t like driving the tesla so much these days 🤪

I expect whether you can add to a garage CU depends on its fuse rating and diversity (is there a home battery charging at night too?)  and what your sparky thinks.

Posted by: @robl

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whether you can add to a garage CU depends on its fuse rating and diversity

That's absolutely the case.

I haven't (yet) suggested that @editor tells us the cable size/type and distance between the main house CU and the one in the garage.
If it were substantial, it may indeed offer a route to run an EV charger off the garage unit.

There are formulae for calculating this, which also take into account the ability of that inter-connecting cable to dissipate heat.

Here on the forum I tend to first suggest a strategy which demonstrates best practice,
rather than what might 'just about be acceptable' for a particular site.

Yes we have the Wiring Regulations, and the quality of work by the majority of British electricians is very high.

But if we can do better than the basic regulatory requirements, then I think we should do so!

Posted by: @robl

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The larger one is fed with 6mm2 cable from its own tiny CU [....] which is fed from the main tails using henley blocks.  Don’t know why the sparky used Henley’s rather than coming from main CU, there was room. 

So we all know - this is a Henley block

It allows three or more cables to be connected together outside of a consumer unit.
Most are black, but Proteus offer colour-coded Henleys.

They are too often fitted incorrectly.
Typical mistakes are:

• Henley block itself not firmly screwed to the backboard
• copper conductor gripped by only one of the internal bolts
• the brass bolts not sufficiently tight (creates heat)
• the lid-screw not 'sealed' with a wire loop in circumstances when it should be (when it's used with meter tails for example)
• the outer insulation of a meter-tail cut back short, which exposes the inner insulation layer

Issues arise when it's unclear if the Switch Disconnector in the main Consumer Unit will or won't remove power from adjacent Henley Blocks.

I'd need to see a photo of Rob's installation to comment further.

Have you considered making space in your consumer unit by using RCBO's instead of RCDs and MCBs?

Since I had dug a trench for the heat pump only for Octopus to say it had sent the wrong diagram, I decided to get Octopus to fix me a Zappi for my EV. I had previously had the House CU replaced with 5 spare slots for these works and was surprised that the Installer brought a small Proteus CU. He said they always fit their own to avoid responsibility for subsequent faults in the existing house one. Any alterations to a CU renders the electrician responsible for the whole installation. He also found the Smets 2 smart meter the other installer fitted had 16 mm tails not 25 mm , he sorted this out while tidying the board. Next the ASHP people will fit  two more CU's!! The unusual thing I was taught about electrics is that in the event of a serious accident you are guilty until you prove your innocence!!

Posted by: @jancold

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He also found the Smets 2 smart meter the other installer fitted had 16 mm tails not 25 mm

It's quite possible that the incomer to your house is (still) 16mm² of course.

When we had appliances which only switched on periodically, that was regarded as acceptable, and was often connected to a 100A service fuse.

But we now have devices which draw high current for extensive periods of time, and you'd expect 25mm² wire to be the norm.

Fitting a separate CU for the EV trips is ok, provided it's clear whether or not this is energised using the Switch Disconnector on the main CU.

An electrician might decide to use separate Switch Disconnectors, or possibly rely on an isolator on the tails between the meter and the two CUs.

I think it best to label everything whilst it's still fresh in your mind.

Posted by: @bontwoody

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Have you considered making space in your consumer unit by using RCBO's instead of RCDs and MCBs?

That depends on whether your EV charger requires Type-A or Type-B  RCD protection.
There are no RCBOs with Type-B (DC fault detection).

Type-B RCDs are obviously 'better', but fearfully expensive.

When I bought my Chint NL210-63-463/30 RCD, the list price was above £250 +VAT.

As it's a safety device, the manufacturer and where you get it from matters.
I can import a Chint RCD type-B from the Far East for £50, but the only thing 'Chint' about it is the printing on the front.

I wouldn't trust it to save my life in the event of a fault. 🤔 

@transparent I had two RCDs in my consumer unit and have just taken out one and replaced the MCBs it was supplying with RCBOs. That created two empty slots. I suppose you could keep the car charger on the remaining RCD and just move some other circuits to the RCBOs

Yes @bontwoody

but it depends what that RCD is.

If it's just a standard AC-rated unit, then it's unlikely to be suitable for an EV charger.
They're renowned for imposing DC leakage currents.

As I understand it, if a slowly rising common mode DC is passed through the AC only type of RCDs then it will saturate the core in it, and then it can’t detect any signals.  This means that not only will that dc not trip it, but not will any subsequent ac fault current.  This is especially critical for some fuse boxes which have one RCD covering many MCBs - if any of them have a small dc offset, then none of the circuits will have RCD based protection.  I’m not sure which electronics is most likely to impose dc fault currents though - why are car chargers worse than high power computer power supplies, which have similar high power switch mode systems.  In contrast resistive loads and old fashioned 50Hz transformers won’t have this dc fault current.

@transparent

"But we now have devices which draw high current for extensive periods of time, and you'd expect 25mm² wire to be the norm."

Agreed and my understanding when I changed my CU many years ago was that nothing less than 25 mm tails were acceptable then, let alone now, and the DNO cable supply size was up to them, not for us to even know about. Maybe the meter replacement installer knew that as he had arrived with the isolating switch and meter pre-wired to connect to the cut out fused at 100A. I really think we deserve to know more about how our electric supply works and is connected  thank you @transparent .