On an open tarred road in good conditions, what following-distance rule of thumb does Highway Code Rule 181 give?

1. One metre for each km/h of your speed (60 km/h → 60 m; 80 km/h → 80 m)
2. One metre for every 10 km/h (80 km/h → 8 m)
3. Always 10 metres, regardless of speed
4. The same as your thinking distance only, ignoring braking