The Working Time Regulations define working time as any period during which the worker is:
That might seem to rule out workers on standby or on call, but there have been a number of decisions in the European Court of Justice to the effect that periods when a worker is on call at the employer's premises, rather than just contactable, count as working time for the purposes of the Directive and therefore for the UK Working Time Regulations (the UK legislation which implements the European rules).
This ruling seems to exclude those workers who are simply available awaiting contact from the employer, say at home, while still able to carry on with family or private life. These are termed 'non-resident on-call' duties. In this case, only the time actually worked will normally count as working time.