Motorway Services (Rear Access)

Anyone have an insight on the policy around the use of the motorway rear access. I am travelling to Halesowen tomorrow and there is no destination charging at my location but it's only 3.1 miles to the Frankley services Ecotricity charger using the 'rear entrance' and 12.8 miles using the motorway exits. The signs explain that there is no access to the M5 but since I would like to be able to only access the services arriving and leaving by the rear access I was wondering what everyone's interpretation of this since I wouldn't be using it as a short cut to the motorway.

This is a 6 mile vs 24 mile diversion there and back

I will very likely just use the motorway access anyway but I may give the services a call first to see if they mind me accessing the rapid charger via the local road. Worst they could say is no.