Some traffic light controlled junctions have an advanced stop line (ASL) to allow cyclists to be positioned in front of other traffic - please see the diagram below:
The Highway Code: using the road (159 to 203)
If the traffic lights are on red, drivers (including motorcyclists and scooter riders) must not cross the first stop line - if they do they could liable to a £100 fixed penalty and three penalty points on their driving license.
If the lights change from green to amber as a driver (including motorcyclists and scooter riders) approaches but they cannot safely stop before the first stop line, they can cross the first line but must stop before the second stop line. In these circumstances it is not an offence to stop in the marked area.
Drivers (including motorcyclists and scooter riders) should avoid blocking/encroaching onto the marked area at other times e.g. when the junction is blocked.
Note that just because there's a car in the ASL box does not mean to say the driver has committed an offence. The offence is only committed when the vehicle enters the ASL box when the light is red. If the vehicle enters the box and the light changes to red, no offence is committed.
Cyclists must not cross the second stop line while the traffic signal is red. Contravening a traffic signal is against the law, and could result in a £50 fine.
Some local authorities have run publicity campaigns with slogans such as: The bike box - get behind it.