Series I Land Rovers were born at the same time as the radial tyre (Michelin patented the radial design in 1946). The Rover company however used cross-ply tyres rather than go with the new design because of their stronger sidewalls and they were considered to be more viable in a farmer's field. Cross-plys could also be more easily repaired in remote overseas locations when punctured.
Land Rovers less than 20 years old are capable of more sustained high speed than a Series Land Rover and should therefore be fitted with radial tyres. These tyres have more resistance to general wear and they are more resistant to heat and contribute to better fuel economy.
A Series Land Rover riding on radial tyres has more steering control when cornering; increased flexibility in the tyre wall causes more tread to be in road contact compared with cross-plys.
Ideally, if you are choosing to use radial tyres then your Series Land Rover should have a full set of radial tyres, but by law (in the UK), you can fit cross-plys on the front and radials on the back. You cannot do the opposite and have cross-plys only on the front as this causes steering instability and is therefore dangerous. Neither can you mix cross plys and radials on the same axle.