Thanks for the kind words - relatively old person here . . .
Either replacing the existing CU, or fitting a small additional CU for the garden office are OK as far as the regs go. If you choose to change the main CU for a larger one, then it needs to comply with the current regs, essentially a metal one, with RCD protection to all circuits, with only minor exceptions, like alarm circuits.
Looking at what's needed for a new CU for the whole installation, then you have a couple of options with regard to residual current/earth fault protection. You can opt to fit a split board, that has one half of the installation protected by one RCD, the other half protected by a second RCD. The advantage of this is primarily cost, it's usually the cheapest option. The disadvantages are that you still lose half the circuits if there's an earth leakage fault somewhere, plus there is a risk that you may get nuisance tripping. The latter point is becoming more of an issue, as we have more devices that have an inherent earth leakage current. Pretty much all devices that use inverters or switched mode power supplies introduce a bit of earth leakage, usually through the interference suppression circuits. Normally this doesn't matter too much, but with enough appliances like this connected it is possible to get up to the leakage current at which a 30 mA RCD will trip, which may well be around 22 mA to 25 mA. For example, the normal earth leakage current for our house installation is around 19.5 mA, so it wouldn't take much more to get an RCD to trip.
One way around this is to fit RCBOs, in place of two RCDs and a bank of MCBs. This is more expensive, but does mean that each circuit is individually protected against both over-current and earth leakage faults. The advantages are that the cumulative normal earth leakage will now be spread across several RCBOs, significantly reducing the probability of nuisance tripping, if a circuit does trip, all the others keep working and you know roughly where the problem is, and finally, an all-RCBO board can be more compact, as there won't be a couple of big RCDs in it. The disadvantage is really just cost, as compact RCBOs are more expensive than MCBs by a fair bit. FWIW, I fitted an all-RCBO board when we built this house, as in my view it's the best solution (it also means I'm biased). One requirement in the 18th Ed is for the leakage current to be checked, primarily to determine if there might be an issue with future nuisance tripping. The maximum permitted value for any one RCD is now set at 30% of the nominal trip current, so just 9 mA for a 30 mA RCD, or a total of 18 mA for a typical dual RCD board. Some installations may exceed this, in my experience (ours would, for example).
Moving on to surge protection and arc fault protection, then both are recommended, but not necessarily mandatory. In the case of surge protection, the installer is supposed to survey the installation and do a risk assessment. This is detailed in section 443.5 of BS7671:2018, and depends on your location (because it uses a lightning flash density map) and also needs the length of the supply cable to the property. If the Calculated Risk Level (CRL) is > or = 1000 then a Surge Protection Device (SPD) is not required, less than 1000 and one is required. I doubt that any electrician bothers to do this risk assessment, they will probably just fit a board with an SPD as it's easier for them. The advantage of an SPD is that it may provide a degree of additional protection to voltage sensitive equipment for the rare times there might be a lightning flash nearby, although often decent equipment will already have this protection built in. The main disadvantage is that surge protectors have a finite life, so need replacing periodically, typically about every three years. My personal view is that I'm on the fence about fitting them to a board unless the CRL indicates an SPD is needed. I doubt many people will check them, so in all probability they may well stop working after a time, anyway.
Arc fault protection, provided by an AFDD, is recommended to mitigate thermal effects from arcing within connections or appliances. Probably of more relevance to installations with a fair bit of switchgear, but perhaps worth considering. It's only a recommendation for domestic supplies at the moment, and given that, for a domestic installation, the most likely cause of arcing is probably a poor connection somewhere in the installation itself (loose terminal screws, etc) then my view is that it's better to concentrate on regular inspection and test of the installation, such that any potential arc sources are detected and corrected before they present a problem. AFDDs are an additional level of detection aimed at installations that may not be inspected as regularly as they should (there is a requirement that domestic installations are inspected and tested not less than every ten years). I'm biased again, as I tend to inspect and test our installation every four or five years, so my view is that an AFDD isn't necessary, but I do know from experience that there are many installations around that go for decades without any form of inspection or testing, so maybe an AFDD does make some sense for those.
For protection for the Qubev charge point, the one with integral open PEN and DC residual current protection, then it needs upstream over current and residual current protection, but this can be just a Type A RCD and Type B MCB, or an RCBO with those characteristics. If connecting the charge point to an existing CU, that already provides RCD protection, then only an MCB should be used, as it's good practice to ensure that RCDs in series have discrimination. This means, for example, that an installation that has a 100 mA trip current RCD on the incoming supply (commonly found with TT installations) can have 30 mA trip current RCDs protecting circuits downstream, as the differing trip currents provide discrimination (the 30 mA one will trip before the 100 mA one).
If you decide to fit an additional small CU, fed from the incoming supply, then a 40 A RCBO is fine. If you choose to feed the charge point from a new CU, that already has RCD protection, then all that's needed for the charge point is an MCB, as it isn't good practice to have two 30 mA trip current residual current devices in series (hope this makes sense!). If you opt to fit a new CU, and also opt to make it an all-RCBO units, then as long as you use double pole RCBOs, you can just add one to supply the charge point.
On the topic of single pole versus double pole, then double pole, or at least 1P+N, RCBOs are very much better than single pole RCBOs. There are two or three suppliers of DP miniature RCBOs now, and they make for a neat and compact installation, as well as providing enhanced safety, by isolating both the line and neutral to each circuit. 1P + N RCBOs isolate both line and neutral, but only sense current on the line side. In my view that's fine in a domestic installation, as there's not often a situation where the overload current in the neutral will be greater than that in the line.
When it comes to recommendations for what brand etc to fit, then a few years ago I'd have said Hager was probably the best. Recently there's a relative newcomer that's come along and is producing some very well designed consumer units, Fusebox. I looked at one of these a few months ago, on a new build, and was very impressed. Easy to install, and not too expensive. An unpopulated 13 way all RCBO board, with isolator switch and SPD, is around £90. They do some very neat 2 pole RCBOs to fit this board for about £18 each. I've no connection with them at all, I just happened to see one and thought it was a nice bit of kit.
