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The law is quite clear on it and in a test it is treated the same as not stopping for a red:
Legislation
The Traffic Signs Regulations and General Directions 2002 deal with light signals for traffic and pedestrians.
Regulation 36 – significance of light signals
Regulation 36(1)(c) states that the amber-with-red signal shall not alter the prohibition conveyed by the red signal.
Regulation 36(1)(e) states that the amber-alone signal shall convey the same prohibition as the red signal, namely that vehicular traffic (other than tramcars) shall not proceed beyond the stop line, except that, as respects any vehicle (other than a tramcar) which is so close to the stop line that it cannot safely be stopped without proceeding beyond the stop line. In such circumstances, it shall convey the same indication as the green signal which was shown immediately before it.
Regulation 36(1)(a) states that the red signal shall convey the prohibition that vehicular traffic shall not proceed beyond the stop line.
Regulation 43(1) defines the term “stop line” in relation to light signals as the road marking placed on a carriageway in conjunction with those signals.
Offence
Non-compliance with Regulation 36 naturally constitutes an offence contrary to s.36 of the Road Traffic Act 1988 (Drivers to comply with traffic signals).
In addition, the prohibition on passing over the stop line applies to any part of the vehicle when the red light is showing; if the front of the vehicle has already crossed that line when the light goes red, it is an offence under s.36 for it to proceed further.
Penalties
All offences under s.36 of the Road Traffic Act 1988 will involve a notice of intended prosecution. Once this has been returned the following penalties are available:
3 penalty points; and a £60 fixed fine, but if the matter proceeded to court the maximum fine is £1,000.
You're both wrong. The junction light timings take into acount the size of the box. Yes the driver is in the lane (which the red appies to) but carrying on through an amber is legit if it's not safe to stop. If the driver looked in his rear view and decided it wasn't safe to brake hard (impossible for us to know) then I think that is not even a minor in a test.
Hopefully there is an instructor on hear who'll know for sure.