Motorway Service Stations

Hi, we have just begun our caravaning experience this year and are venturing on our 2nd Trip to cornwall which will be over a 6 hour trip with our dog.  We are planning on setting of early to avoid Saturday traffic and thought we would sit out the waiting time for accessing caravan site at 1pm at one of the service stations down that way.  Could anyone recommend which one would be best that allows caravans, we got caught out on our last trip to the lakes as it didn't allow caravans.  Thanking in advance for any suggestions.