This is the schedule Gareth referred to in his post, thought it might be worth posting.
It’s schedule 6 part (I) of the 1984 RTRA and refers to vehicles fitted with pneumatic tyres.
The Road Traffic Regulation Act 1984 provides traffic authorities with the authority to regulate traffic within their area. Part I of Schedule 6 to the Act imposes maximum speed limits on vehicles fitted with pneumatic tyres on all wheels on various types of road.
Item 1.A passenger vehicle,
motor caravan or dual-purpose vehicle not drawing a trailer being a vehicle with an unladen weight exceeding 3.05 tonnes or adapted to carry more than 8 passengers, is limited to a maximum speed of:
70 mph whilst being driven on a motorway,
(60 mph if the overall length exceeds 12 metres)
60 mph on a dual-carriageway not being a motorway, and
50 mph on any other road.
Item 2.An invalid carriage is limited to a maximum speed of 20 mph.
Item 3..
A passenger vehicle, motor caravan, car-derived van or dual-purpose vehicle drawing one trailer is limited to a maximum speed of:
60 mph on a motorway or dual-carriageway, and
50 mph on any other road.
Item 4.A passenger vehicle, motor caravan, car-derived van or dual-purpose vehicle drawing more than one trailer is limited to a maximum speed of:
40 mph on a motorway, and
20 mph elsewhere.
Item 5.A goods vehicle having a maximum laden weight not exceeding 7.5 tonnes and which is not an articulated vehicle, or drawing a trailer, or a car-derived van is limited to a maximum speed of:
70 mph on a motorway,
60 mph on a dual-carriageway not being a motorway, and
50 mph on any other road.
Item 5(2)(a).A goods vehicle which is an articulated vehicle having a maximum laden weight not exceeding 7.5 tonnes, or a motor vehicle, not being a car-derived van, which is drawing one trailer and the aggregate maximum laden weight of the motor vehicle and the trailer does not exceed 7.5 tonnes is limited to a maximum speed of:
60 mph on a motorway or dual carriageway, and
50 mph on any other road.
Item 5(2)(b).An articulated vehicle having a maximum laden weight exceeding 7.5 tonnes, a motor vehicle having a maximum laden weight exceeding 7.5 tonnes and not drawing a trailer, or a motor vehicle drawing one trailer where the aggregate maximum laden weight of the motor vehicle and the trailer exceeds 7.5 tonnes is limited to a maximum speed of:
60 mph on a motorway,
50 mph on a dual-carriageway not being a motorway, and
40 mph on any other road.
Item 5(2)(c).A motor vehicle, other than a car-derived van, drawing more than one trailer is limited to a maximum speed of:
40 mph on a motorway, and
20 mph elsewhere.
Item 6.A motor tractor (other than an industrial tractor), a light locomotive or a heavy locomotive:
(a) if the provisions about springs and wings are complied with and the vehicle is not drawing a trailer, or if those provisions are complied with and the vehicle is drawing one trailer which also complies with those provisions the maximum speed limit is:40 mph on a motorway, and
30 mph on any other road.
(b) in any other circumstances the maximum speed limit is 20 mph on all roads.
Item 7A works truck is limited to a maximum speed of 18 mph on all roads.
Item 8.An industrial tractor is limited to a maximum speed of 18 mph.
Item 9.An agricultural motor vehicle is limited to a maximum speed of 40 mph on all roads.
NOTES:
This Part applies only to motor vehicles, not being track-laying vehicles, every wheel of which is fitted with a pneumatic tyre and to such vehicles drawing one or more trailers, not being track-laying vehicles, every wheel of which is fitted with a pneumatic tyre.
A "Car derived van" means a goods vehicle which is constructed or adapted as a derivative of a passenger vehicle and which has a
But these are the rules chaps, disobey them at your peril.