Signs in off-street car parks don’t have to conform to TSRGD. The authority is sensible to have signs that are similar to on-street ones that people will recognise, but there’s no problem changing the panel order and adding the extra text.
But two interesting problems with requiring the vehicle to be actually charging are:
(1) how soon after it is fully charged does the motorist have to remove the vehicle?
(2) what evidence do you need from the charging apparatus to show whether charging is in progress or not?
In practice, it is normally assumed that a vehicle connected by a cable is charging. But you might need a provision for preventing the same vehicle hogging a space for days on end!