I don't see how a horse and cart could be made to pay the congestion charge even if it were technically liable for it. After all, the charging system works through numberplate recognition cameras, and it works automatically. There's no operator sitting there scribbling down the numbers of each car that goes past. I can't see them watching the cameras and saying, 'Yep, that's the one, the fat little skewbald with two blokes in the cart behind ... just heading into Chancery Lane now. Send a traffic warden down there to collect the £8 or stick a parking ticket on the horse's arse.'