Two Horse Power.

"Before riding off or turning, look behind you to make sure it is safe, then give a clear arm signal. When riding on the road, you should:

Keep to the left
Keep both hands on the reins unless you are signalling
Keep both feet in the stirrups
Not carry another person
Not carry anything which might affect your balance or get tangled up with reins
Keep a horse you are leading to your left
Move in the direction of the traffic flow in a one-way street
Never ride more than two abreast, and ride in single file on narrow or busy roads and when riding round bends."

- The Highway Code