According to the VIRM it doesn't appear to be a reason to fail.
Lighting: 4-14 Reversing lamps
Reasons for rejection
Permitted equipment
1. A vehicle is fitted with
the rear of the vehicle.
2. A retrofitted pair of reversing
a) symmetrically mounted,
b) mounted as far towards
practicable.
Condition
3. A lamp is insecure, obscured or contains visible moisture or dirt.
4. A lens is missing, or has a hole, crack or other damage that allows moisture or dirt to enter.
5. A reflector is damaged or has deteriorated so that light output is reduced.
Performance
6. A lamp controlled by gear engagement continues to display a light to the rear when the reverse gear is disengaged.
7. A lamp controlled by a manual switch continues to display a light to the rear while the headlamps are switched on.
8. When engaged, a lamp emits light that is not:a) substantially white (Note 3), orb) steady, orc) diffuse or a dipped beam.
9. Where a lamp comprises an array of light sources (eg LEDs), fewer than 75% of these operate.
Note 1 Definitions
Reversing lamp means a lamp designed to illuminate the area behind the vehicle while it is reversing and to warn other road users that the vehicle is reversing or about to reverse.
Note 2
A reversing lamp that does not comply with equipment, condition and performance requirements must be made to comply or be disabled so that it does not emit a light.