"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 .

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.

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. 🤔 

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.

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!

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.

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.