Another vote for the use of Google Earth - I've always been able to pre-plan rest and refueling stops.
Despite all manner of other vehicles abusing the allocated caravan spaces, be very wary about using coach spaces as many services will issue fixed penallty notices for any non-coaches parked there.
Don't know your route but most motorway services have a 2-hour free parking limit although those operated by Westmorland (Tebay, Cairn Lodge/Happendon and Gloucester) and Cornwall Services on the A30 have a 3-hour free parking limit.