I take great delight in your actions against these motorway service stations as I for one got fined for breaking down in one on way home in my motorhome and despite a massive AA recovery truck, vans x 2 about me all trying to get me going they fined me for being three hrs over, phoned and challenge then asking if they have cameras then why could they not see I could not move and had AA to fix me, they demanded I send them copy of the breakdown paper recipt from the AA and it would be discarded .
As for if its inforceable or not then which do you wish to have a lengthily battle of letters and debt agency try to collect and your black listed wheather your right or wrong , unless you phone and challenge them with a plausible reason and proof that they are unable to inforce such a ticket without your acceptance of who was driver they can't inforce it on you unless they can prove your the only driver owner of vehicle.
Hope I made sense , confront , confuse and don't pay .