The Highway Code rules on speed are clear:
RULE 124 – You MUST NOT exceed the maximum speed limits for the road and for your vehicle. The presence of street lights generally means that there is a 30 mph (48 km/h) speed limit unless otherwise specified.
RULE 125 – The speed limit is the absolute maximum and does not mean that it is safe to drive at that speed irrespective of the conditions. Driving at speeds too fast for the road and driving conditions can be dangerous.
RULE 126 – Drive at a speed that will allow you to stop well within the distance you can see to be clear.
RULE 205 – There is a risk of pedestrians, especially children, stepping unexpectedly into the road. You should drive with the safety of children in mind at a speed suitable for the conditions.