The Highway Code applies to England, Scotland and Wales and is essential reading for everyone.
Rule 161

All mirrors should be used effectively throughout your journey. You should

your mirrors frequently so that you always know what is behind and to each side of you
use them in good time before you signal or change direction or speed
be aware that mirrors do not cover all areas and there will be blind spots.
you will need to look round and check.

Remember: Mirrors – Signal – Manoeuvre