I don't see how the advice applying to primary schools can be applied without adaptation to sixth forms.
It is a totally different structure of education. A primary school class can be readily limited to a bubble.
A sixth form of 200 in year 12 and 200 in year 13 all doing only 3 or 4 subjects in relatively small class sizes is entirely different. The bubble is not really 200 pupils. A pupils will probably only be in a class room for longer than 15 minutes with a few dozen people since they only do 3 or 4 A levels and those are in fixed classes. I guess you can add their tutor group to that.
But if one pupil who does none of the same subjects as another pupil and is in a different tutor group gets a positive test result , it makes no sense for the other pupil (or all 200 pupils) to automatically lose 2 weeks of in-school education. It is perfectly possible to only ask those pupils who were actually in close proximity for longer than 15 minutes to stay at home.