Driz, we’ve been advising several councils over this.
The traffic sign is vague as to whether you have actually have to be charging or not. As you say, it’s down to what’s written into the traffic order and what enforcement practice the authority uses. Most consider that there must be a cable linking the car and the charging point, but for CEOs to have to interpret the lights to determine if charging has finished is a step too far at present. Plus it presents the difficulty that a legally charging car would suddenly become illegally parked when the charging was complete. That might be in the middle of the night, for example, when the owner couldn’t be expected to move the vehicle for several hours.
So setting a maximum length of stay seems the most sensible way of getting a turnover of space usage, but it might need to be a long maximum for the low wattage charging points (often retrofitted onto lamp columns) that you mention.
Whilst it’s not shown in TSM Chapter 3, TSRGD Schedule 4 does allow you to add a maximum length of stay to the vertical sign for a EV charging point.