Would 6 on 2 off forever be legal?

The Enforcer:
Yes, I would agree, taking my rosta as shown you would have a week in the rosta without a weekly rest attached to it.
However whilst the weekly rest shown in week 11 appears to only cover mon & tue in reality it would start when your shift ends on Sunday meaning you can attach it to the previous week.

The only difference between the diagram you’ve put up and the one I’ve put up is that you’ve counted week 2 as the first working week whereas in the hope of making it a bit clearer I’ve started with the first working week as week 1.

In my diagram the weekly rest period at the start of week 10 can be used for week 9 or week 10 but not both, therefore one of those weeks has no weekly rest period, in your diagram it’s weeks 10 or 11 that has no weekly rest period.

Either way it shows that (disregarding bank holiday and other leave days) a 6 on 2 off shift pattern does not work permanently without having extra time off.