The Motorway services act says rear access are not legal entry exit points for the motorway network and hence should be controlled access not for customers/motorway users, this also applies to cross over bridges. They are provided as access points to the services only, not as entrances/exits from the motorway network. A services manager may allow discretionary access to cross the bridge as long as you return to your original side of the motorway.
more here: http://motorwayservicesonline.co.uk/Rear_Access
more here: http://motorwayservicesonline.co.uk/Rear_Access