It depends. The Working Time Regulations define working time as any period during which the worker is:

• working;
• at the employer's disposal; and
• carrying out the worker's activities or duties.

Whether or not you are working or at rest when you are on standby depends mostly on the extent to which that time is under your control, as opposed to that of the employer. It depends, for example, on the extent to which your employer can control where you go, what you wear, and how quickly you must respond to a call out. If, for example, you are allowed to stay in your own home, or move around freely when on standby, you are unlikely to be working. By contrast, if you are required to stay inside a narrow geographical area and to be ready to respond to a call out at very short notice, your time is much more likely to be ‘working time.’

The European Commission has been consulting for some time on changing the rules concerning 'on-call' time and its effect on the rules for compensatory rest, including a year-long online public consultation from 2014-15. The extent to which the outcome of the consultation will affect UK workers will depend on the eventual outcome of Brexit negotiations, once these get underway.