What's a good post code to use for Frankley rear access?You shouldn't have a problem going in and out of the Frankley car park from the rear access. My understanding of the rules is that you're not allowed to use the rear access as a short cut to or from the motorway. If you go in and out of the rear access you should be fine.
I stayed overnight at Frankley recently (southbound). There is no barrier etc so you can just drive straight in (as I observed others doing). I think they use number plate recognition cameras these days.