Ewan,
If you are going down via the M74 and M6 the caravan site attached to the north bound services at Tebay would be worth considering for an overnight stop. Although on north bound side it is still accessible via a service road from southbound service area. Saves any deviation from main road but you are parked in a "real" site away from lorries etc. Level pitches, all hardstanding,toilet and shower block, and well screened by trees. We use it quite often when heading south, it is only two and a half hours drive from where we live but gets us off on an early start to wherever we are off to.
Robert