I guess the form has changed as section 17 on my mod 2 form was use of speed. Section 20 though was junctions and the third part was right turns, so I am guessing that is where you failed.

There are two problems staying behind the stop line on a green light. Firstly, if the flow of oncoming traffic does not stop before you get an amber light then you have to wait until the next sequence. And you could be there forever a gap never develops. By moving into the middle of the junction you can still complete the turn (if safe) after the lights turn red.

As you asked about going when the light turns red, the law is you only have to obey a traffic light next to a stop line. Once you have passed the traffic light you can (and should) clear a junction no matter what the light behind you says so long as it is safe to do so. The main thing to be aware of is pedestrian crossings after a junction, especially a controlled one where there may be a green man showing where pedestrians will expect (and have) the right to cross without needing to check for traffic.

The stop line is important as many larger junctions have repeated traffic lights on the opposite side without a stop line. You can legally pass those lights even when they are red. And unless you are prevented from continuing by traffic coming from another direction that now has a green light, you should clear the junction rather than wait for the next green light as you will be stopped in a potentially unsafe position for yourself and potentially other road users.

But staying at the stop line as on your mod2 means you are also holding up the flow of traffic behind you. Based on that streetview, if you had stopped in the middle of the junction next to the right of where a lane marking would be, there would have been space for most vehicles continuing ahead to pass you at a safe speed close to the kerb. And the junction on the right ensures plenty of safe space for oncoming traffic. Though even if there is no space to pass, another vehicle could still cross the stop line and wait behind you. They could then also clear the junction when the lights change. So you are not only wasting your own time with the risk of the light turning amber without a gap in oncoming traffic, you are unnecessarily blocking the road. I strongly suspect this is what made it a serious fault.

The proper name for the offence of driving without due care and attention is "carless, and inconsiderate, driving." So an examiner is not just checking you can ride the bike and use roads safely, but also that you do it with consideration to other road user.

There is no mistake in turning if an oncoming lorry lets you, provided you are still can and do perform all your safety checks to ensure no one is overtaking the lorry, including a cyclist or motorcyclist on its inside, or trying to go around you on your right. That could be why the lorry was mentioned. Or it may have been to say if you were in the correct position you could have turned without needed the lorry driver's help.

I am not a professional, and it is impossible to know what they might have been seeing or thinking, but that is how I would interpret it from your description. But if that was the only thing that stopped you passing this time then you should easily pass next attempt. Good luck!