Can a cupboard under the stairs be turned into a WC if the electric meter is in there?

The meter doesn’t stop it being a W/C so long as it’s covered properly. The biggest problems will be ventilation and pipe work. ——— The ventilation would be for the room, an extractor fan or window is required. I assume you already have one or both as you already have the toilet. The boxing for the meter needs to be made of noncombustible materials ie metal or cement board. You must be able to access the meter along with all the connections, so a door or hatch will be required that will also need to be made of noncombustible materials.

Yes it can as long as there is still good access to the consumer unit. In my experience I’ve seen a lot done in such a small space. Sinks on the back of toilet systems etc. Sewage and water pipes if not easily accessible into that area will be the biggest concern

I helped a client do such a conversion not so long ago. If the meter(s) and or supply heads are in the way then they can be moved but it all gets a bit involved / expensive the more that has to go. In my customers case we moved the electric head & meter to a cabinet outside and the Consumer unit (renewed) to a new position inside. The gas stayed where it was and he boxed around it. Although there is no requirement for forced fixed ventilation, its never a bad idea!

The special locations that would interest you in the regulations are ones with a bath or shower, so not this (there are loads of other special locations including fairgrounds even!). However there is a regulation about good access to the unit, so the boxing in you do must allow usable access to the board, earthing connections, cables and main fuses. So that's being able to open the unit up and access to the cables and connectors. However the humidity and ventilation will be a different matter, hopefully it's on an outside wall to make the ventilation access easy. It's the loo that would need ventilation, rather than the fuseboard/meter. They should have had building regulations approval for the toilet though, as it's notifiable work?