The other driver could be at fault for making an unsafe overtake.
140 No overtaking unless safe to do so
A driver must not overtake a vehicle unless —
(a) the driver has a clear view of any approaching traffic; and
(b) the driver can safely overtake the vehicle.
44 Keeping a safe distance when overtaking
A driver overtaking a vehicle —
(a) must pass the vehicle at a sufficient distance to avoid a collision with the vehicle or obstructing the path of the vehicle; and
(b) must not return to the marked lane or line of traffic where the vehicle is travelling until the driver is a sufficient distance past the vehicle to avoid a collision with the vehicle or obstructing the path of the vehicle.
There is also this rule
126 Keeping a safe distance behind vehicles
A driver must drive a sufficient distance behind a vehicle travelling in front of the driver so the driver can, if necessary, stop safely to avoid a collision with the vehicle.
Although the reversing rule does also apply in your case, the other driver can still be at fault.
Parallel parking is one of the most challenging parking maneuvers. If you haven't already, learn the basic parking skills before learning to parallel park.
Getting started
If you are worried about bumping into another car, start off on a street or in a car park where it's safe and there is little or no traffic around. You're not likely to have a serious crash while parking but small crashes can be expensive. Don't try your first parallel park in a busy street.
Practice parallel parking
Check your mirrors.
Before you intend to park, check the rear and near side mirrors for traffic behind. If you have vehicles following you closely and will not be able to slow down smoothly and safely, keep going and choose another place to park.
Use your indicators.
Give other road users plenty of warning that you are going to park by indicating early.
Slow down.
Brake gently and slow down gradually. Check that the drivers behind you are responding.
Be observant.
As you approach the vehicle you are parking behind, notice its size and how far out from the kerb it is. Later, this information will be useful.
Prepare to park.
Come almost to a stop along side the space you intend to park. (This forces any drivers behind you to hang back and gives you room to reverse.) Ease off the brakes and move slowly forward alongside the car you are going to park behind – keep a good arm's length out. Stop when the rears of both cars are level.
Reverse into the parking spot.
Select reverse gear. Look back over your shoulder into the parking space and keep looking back. Reverse slowly until you judge your back wheel is getting near the rear of the car next to you. (Your back wheel is what's called a ‘pivot point’ - the point your car pivots around as you steer.)