It depends what type of vehicle you're driving.
Single carriageway = 60mph in a car/van, but 50mph when towing a caravan or trailer (or when driving a bus/HGV).
Dual carriageway = 70mph in car/van,but 60mph when towing a caravan or trailer (or when driving a bus/HGV).
HGV is 40 & 50 respectively, 60 on a motorway.