As a former employee at an un-Welcome Break service station, I can confirm that their signage is as useless as the rest of their company.
As far as I was aware from my training there, only service stations not ‘on’ motorways can legally sell alcohol as it is illegal to sell (or consume, obviously) alcohol ‘on’ a motorway. Just in case any naughty people were tempted to drink it whilst driving on the motorway! Although if you eat the food at a service station you could be just as dangerous driving…