Rule 202 of the Highway Code:
Look carefully before you start reversing. You should
use all your mirrors
check the ‘blind spot’ behind you (the part of the road you cannot see easily in the mirrors)
check there are no pedestrians (particularly children), cyclists, other road users or obstructions in the road behind you.
Reverse slowly while
checking all around
looking mainly through the rear window
being aware that the front of your vehicle will swing out as you turn.
Get someone to guide you if you cannot see clearly.