Which chargers at Gretna Green? When I was there about ten days ago there were (I think) 2 Gridserve chargers with 4 connectors (and one of them broke down mid-charge while I was on it), but there were also six Applegreen chargers with 12 connectors. It was the busiest place I charged at, but even then maybe only half the chargers were occupied. When I couldn't get the AppleGreen charger I tried first to connect I saw a space at a Gridserve charger and was able to move to it immediately. I think someone said there are also some Ionity chargers in a different location in the service area.
My first motorway charge (first public charge not counting the village charger at the end of my street) was at Southwaite, and I was the only EV that used the chargers there the entire time I was there - lunchtime on a Tuesday. I could have done with company when I couldn't figure out how to make the charger let go of my car, but I figured it out in the end. (I had been so slow the charger thought it had a new customer, and I had to re-present my card, feign starting a new session, then abort it. That worked, and it let go.)
While I was on the motorway (M74 and M6) and not in a hurry, I drove into several service stations with no intention of charging, just to figure out where the chargers were and how busy they were. Some were very badly signposted, but most were in the part of the car park nearest the food court. I didn't see a single one that didn't have a connector available, out of the five I investigated.