If the sole purpose of the vehicle is to travel to rallies, not carrying paying passengers or carrying a load for delivery, then what you have got is a private vehicle - just as a converted 'motorhome' is not a goods vehicle.
I'm not sure about the legality of 'dispensing' drinks, but, IMO (unqualified) opinion, if you are just serving your friends (and not charging them) it is no different to entertaining from your motorhome.
http://bcv.robsly.com/vdv135s.html