Is it in fact a power shower i.e. a shower pump ? This type need a bit of water flow to start the pump so you opening a tap causes a shock wave in the pipe causing the pump to start. You need to check why there is not enough flow to trip the switch to start the pump. Isolating valve not fully...
Surely , it depends on the size of cable already installed for the EV charger ? If there is spare capacity then you can T off it for a garage CU. Or put a larger CU on the end of the existing cable with positions for the EV charger, power sockets and lighting circuits.
Please give details of your hot and cold system. Are they both from a tank or do you have a combi boiler or some other system ?
A traditional boiler would have a hot water cylinder etc
One solution is to use a timer fan with live supply (as normal) but use a separate "momentary" pull switch to trigger it (Instead of the light switch). The momentary pull switch only provides a live when being pulled but switches off when released. This triggers the timer fan which then runs...
We found the outside humidity (raining) set the fan going as the fan was on an outside wall so short duct. Increased the humidity setting but then it didn't come on quickly enough during shower.
So we just use the timer mode.
If you are very patient you could cut the elbow off without touching the pipe inside and then sandpaper the pipe ready for a new elbow.
There may be a way to un-weld the glue ?
These days you have to declare things when selling a house and provide a gas safe certificate : you could get a second opinion but looks like a new boiler to me. 18 years is pretty good for modern boilers as they are more complex.
Look around eBay etc - there are some "new" old stock gas valves around. Some people bought a spare and never needed it !
Check the part numbers and photos.
PS Is that the manufacturer part number ? Should have a bit more detail manufacturer's name etc,