DRL activate automatically when the engine is started and remain on unless the headlamps are switched on.
They may however remain off in some low speed and stationary situations. In general all other lights should remain off when the DRL are on but for a limited period some vehicles may be available which automatically activate the rear position lamps at the same time as the DRL.
DRL may be combined with other lamps. For example some manufacturers use the same lamp for both the DRLand position lamp. In these cases the DRL automatically dims to function as a lower intensity position lamp when the headlamps are turned on. DRL placed close to direction indicators may also automatically dim when the direction indicator is active. This is to ensure that the direction indicator is not masked by the light from the DRL.
Source:
Gov.UK