Er, sorry mate, but that don't make sense. You have no speed limit but must be able to do at least 25 mph? If a vehicle is incapable of doing more than 25 mph it becomes a slow moving vehicle and must dispaly amber flashing beacons. Mind you the 'Official' website is about as clear as mud.
Prohibited vehicles. Motorways MUST NOT be used by pedestrians, holders of provisional motorcycle or car licences, riders of motorcycles under 50 cc, cyclists, horse riders, certain slow-moving vehicles and those carrying oversized loads (except by special permission), agricultural vehicles, and powered wheelchairs/powered mobility scooters (see Rules 36-46 incl).
Actually . You MAY drive on a Motorway with a provisonal C C+E or D or D+E licence.