That's not quite accurate. Legally, any new work must be done to modern standards, and the new work must not leave the unchanged parts of the installation in any worse condition than they were before. It is not obligatory to bring the whole installation up to modern standards just as a result of touching one part of it. However, for the specific example of consumer unit changes, any competent electrician will test the whole installation first since it's quite common for old installations to have latent faults that would cause breakers to trip when a modern RCD-equipped consumer unit is fitted. If the new CU was just fitted without testing there's a significant risk of being stuck with no power until the faults can be traced and fixed.
IMO, it would be unwise to do this sort of higher-risk charging from a socket as part of your existing installation with no RCD protection. If you don't wish to upgrade the existing installation, one option is to have a new circuit for the EV charging taken directly from the supply after the meter - the new circuit having its own RCD protection to modern standards. This could apply to either a proper installation of a chargepoint or a circuit for a socket intended for charging.
By 'EVSE' you presumably mean a portable EVSE plugged into a socket of some sort (a normal wall-mounted charging point is also an 'EVSE'). The portable EVSE itself can't require planning permission as it isn't permanently installed. However, the socket into which you plug it could potentially be considered an 'electric vehicle charging point' - the wiring regulations certainly include 13A sockets and commando sockets as options for a "charging point".
OTOH, planning permission is only interested in what is on the outside of your house - if the socket/chargepoint is inside, then it's not relevant for planning permission. In my house for example, the chargepoint is installed on the inside face of the wall of the front porch and isn't visible from outside apart from the cable itself while in use.
The
planning permission limit you are referring to is about 2m from a 'highway' - is your street actually a 'highway', or is it perhaps a private road within your development? OTOH, it also requires the area to be 'lawfully used for off-street parking' - can you lawfully park your car on the bit of gravel?
Another point is that given your awkward circumstances it would be desirable to have the fastest charging you can achieve, so as to minimise the amount of time you need to spend charging - maybe a 13A socket would be enough for your needs, but you would have to charge every night - get a 7kW chargepoint and it charges over 3 times faster, so you only need to charge twice a week.
I suspect that your original suggestion of a separate metered supply to your garage would be excessively expensive to install, and also incurs the standing charges thereafter. However, if there is any power up there then there may be scope for doing a deal with a neighbour to have it from his supply with a (cheap) private meter so you can account for the cost sensibly.
Alternatively, if the garages are detached from all of the houses and none have power in, getting one new supply to give light and power in all of the garages and split the cost between the owners might be feasible: we've done this at another location where there are 12 garages in a block well away from the houses. We got the DNO to provide a supply into one of the garages with a standard meter; a cable then runs along the back of the block into each garage where there's a private meter, lighting and sockets etc and we also provided security lighting outside. Two of the owners didn't want to take part, so they don't get any power; splitting the cost 10 ways made it affordable. This was actually done before EVs were a consideration - electric door openers was what swung it for several people, others wanted to keep a freezer or run power tools in their garage.
