Someone researched and published a pamphlet in the late 90s about alternatives to motorway services. I used to have a copy that is obviously now well out of date. But the author sent me it to aid football fans travelling to away games. Basically they researched food and petrol outlets within 4 or 5 miles radius of motorway junctions. In each case the fuel, food and drink was much cheaper than services rates. They even looked at fast food outlets and compared town centre/retail park with services to reveal the premium being paid for the convenience of a motorway stop.
Obviously it gets more difficult on an evening and oil companies are sometimes wise to pricing near to motorways, But even so the pamphlet showed you could make a considerbale saving by exiting at junctions rather than taking services stops.
It would be really interesting to see an updated survey.