What: Reverse the car into a side road on the left safely and under full control

How: Discuss the routine and consider safety aspects, followed by a demonstration if required and then guided practice

By the end of the lesson: Be able to reverse the car around a left corner safely and with little or no help

Keep the car as slow as possible and under full control at all times

Why: Test / Dead Ends / Wrong Turns
Not possible to do Turn In The Road or U-Turn

Where:

A safe, conveinient and legal place (SCALP) 
Away from pedestrians and children
Away from obstructions (trees, lamposts, other street furnature)
Enough space to reverse into the new road, (At least 2-3 car lengths)

Observation:

360 degree observations before moving  (6 point check)
Your front end will swing out at the point of turn narrowing the road (Check Blindspot)
Use the Left Mirror to guide the car around the kerb (Quick checks) 
Look mainly in the direction of travel with regular checks all around the vehicle
Ensure you understand other road users intentions (Eye Contact)

Control:

Very slow car, effective use of clutch control or brake control
3 parts, Straight reverse - Steer around the corner - Straight reverse
Use a Half Turn, then correct as necessary

Method:

Follow the method described / demonstrated by your driving instructor

Driving The Essential Skills (Pg. 183)
The Official Driving Test (Pg. 37)