Do you happen to know which Motorway Services it is Mike? I used Charnock Richard Services on M6 on my way to and from Nelson last Thursday and I was slightly puzzled by a sign that I hadn't noticed before which read "All alcohol purchased must be consumed on the premises".
I did think at the time that apart from the moral and lawful issues surrounding drinking alcohol on the motorway I imagine that the cost of buying booze at such an establishment would prove totally prohibitive; reminding me of the old Chubby Brown quip "I stopped off at one of those Motorway Services the other day to buy a ham sandwich; well, I only had a tenner on me"