I don't really understand the implications, aside from it being against current regs.
Tutorial on RCDs first, conclusions at the end:
RCDs detect faults by measuring the difference between the current in Line and Neutral - they should be identical, and if different some current has 'leaked' away to earth due to some kind of fault; in the worst case, that leakage is through the body of someone who is touching the faulty wire/car and their feet standing on the ground.
Conventional RCDs (type AC) will only detect where the leakage is 50Hz AC - which is what you get for simple cases where the insulation has failed and the live cores are partly in contact with earthed metalwork, or the wires are bare and a person has grabbed hold of it. These RCDs are very simple to make - just a transformer core with a turn around the two wires and another winding to activate the mechanism. This is all you need for old-fashioned appliances with just motors, resistance heaters etc.
Most modern appliances have electronics in them and will often have a power supply that rectifies the AC supply to generate some sort of DC. If you get a wiring fault inside the appliance, the leakage current is unlikely to be pure 50Hz AC, it's more likely to be pulses of DC (which you can think of as AC at a different mix of frequencies). You need a Type-A RCD to detect this kind of fault. Fortunately, pulsed DC will still activate a transformer, so a Type-A can be built in almost the same way as a Type-AC, just adding a little bit of electronics to the same sort of transformer as a Type-AC. That sort of electronics has trivial cost nowadays, so the Type-A hardly costs any more than the Type-AC. Given the widespread use of appliances with switchmode power supplies, and the minimal extra cost, IMO we should really be using Type-A everywhere - and the industry is slowly moving in that direction, with some brands of consumer unit now being Type-A by default. The regulations have been a bit slow (IMO) in that they have only been requiring Type-A in specific situations of higher risk - but that has included EV charging since 2013. This is sensible as many plausible faults in a typical design of EV charger would generate pulsed DC, and EV on-board electronics are a high risk area (high vibration, risk of un-noticed damage in traffic accidents etc.).
There remains the possibility of a fault that is pure DC without any AC component. Such faults are rare - even if a typical charger/SMPS circuit is generating pure DC at its output, there's not usually any obvious failure mode that will couple that DC back into the supply wiring. However, if you do manage to create a pure DC fault, then it's very bad. Not only will standard Type-AC or Type-A RCDs fail to detect the fault (because transformers don't work with DC), that transformer can become 'saturated' by the DC so that it no longer performs its normal function. This means that the DC fault could sit there un-noticed for a long time (because it doesn't trip anything) and all that time the RCDs are no longer providing their protection against more normal faults (like a person touching bare wires). You need a Type-B (or Type-EV) RCD to detect these pure-DC faults, or a special circuit in the chargepoint to do the same job. Detecting pure DC faults is much more complex since you can't use the transformer trick that magically cancels the L and N currents - it's notable that most Type-B RCDs are significantly bigger than Type-A equivalents to make room of the necessary circuitry. Not £300 worth of extra - that's a premium being charged to reflect the small numbers of these devices currently being made - but could easily be double the manufacturing cost of a Type-A.
Regarding the saturation issue, this can be solved by simply making the transformer bigger - though of course manufacturers don't want to make them bigger than they need to be because that costs money. The specification of Type-A requires it to tolerate at least 6mA of pure DC while still responding to any AC/pulsed DC on top of that. This then sets the specification for the internal DC detection on new-generation chargepoints: if they detect leakage > 6mA and shut down, then that guarantees that any upstream devices won't get saturated and they can continue to be Type-A. This also gives the distinction between Type-B and Type-EV RCDs. Most standard RCDs have a sensitivity of 30mA - being chosen as a compromise to be likely to trip in the event of a current big enough to kill you in a touching-bare-wire scenario, while not tripping on the small amounts of leakage that occur in normal usage (most particularly due to interference suppression components). Type-B RCDs just add DC sensitivity to their existing AC sensitivity at the same level (so typically 30mA) hence they will detect significant DC faults but don't guarantee to keep the level low enough to prevent Type-A devices becoming disabled - going the Type-B route, all your RCDs need to be Type-B. Type-EV on the other hand has general Type-A behaviour (at 30mA), plus DC protection at (6mA) so as to permit upstream Type-A devices. In most cases, EV charging is installed on a separate circuit and there isn't an upstream RCD so the choice of Type-B/EV is unimportant, but sometimes it isn't. One particular case in point is houses or farms with a TT earthing system and a 100mA time-delayed RCD protecting the whole installation against earth faults (that are normally handled by the supplier fuse on other earthing systems). For an installation like that, the Type-EV is more suitable than the Type-B.
So, we have a rare fault possibility and an expensive way of solving it - how does this relate to EV charging?
As above, a pure DC fault inside the car's charger is relatively unlikely - not much more so than your washing machine or phone charger. However, there's a specific failure mode specific to EV charging. The control pilot wire in the charging cable has 12V DC applied to it relative to earth, through a 1K resistance. If the cable becomes damaged and shorts the pilot to neutral, approximately 12mA DC will flow in the neutral. That fault is harmless enough in itself, but the 12mA may be enough to saturate a Type-A RCD. If the cable damage has also exposed the Line wire so you can touch it, now you have an electrocution situation with the RCD disabled. You'd still have to be pretty unlucky to get in this situation, but you have to admit that it might happen.
So, what if you've got an existing Tesla (or other brand installed before 2019) with only Type-A RCD?
- There's a theoretical risk of electric shock from damaged charging cables that your installation doesn't protect you from. You could spend £££ to upgrade, or you could take care to monitor the condition of your cables. If you used to use an electric lawnmower in the 1970's, that was a much bigger risk!
- If it was installed pre-2019, then there's nothing in law pressing you to upgrade; it's a personal decision on the cost vs safety benefit.
- If it was installed 2019 onwards, then any Electrical Installation Certificate claiming that the installation complies with BS7671 is technically fraudulent. You could consider going back to your installer to get it fixed, but if the work was priced on a time-and-materials basis then you will probably end up paying for it anyhow. If it was a fixed price bid and they won it by quoting a lower price by using inadequate materials, then they deserve to be called to account.
