First, let me check that I've read the question correctly:
As I read it. you're referring to the situation where a council has followed the statutory procedures to apply parking restrictions to 'Alpha Road'. There is a side-turning, 'Beta Street', which runs off Alpha Road and which is not subject to parking restrictions. You wish to know how far 'round the corner' the yellow lines can extend.
The first thing which would have to be checked would be the wording of the statutory order which the council had sought, and obtained, to impose the parking restrictions.It's quite common for a parking order, relating to one road, to specifically define how far round the corner, into another road, the restrictions shall apply. Every order can be different and, as long as they comply with the order, the yellow lines can extend to ANY distance around the corner.
Where a parking order does not specify how far around the corner the lines shall extend, there are official guidelines relating to how far the lines can go. These have to take into account that it's not always easy to define exactly where a junction starts and finishes (especially where the side road doesn't form a right-angle with the restricted road). However, as you state, there is a guideline figure of 1.2m.
Anyway, it shouldn't make any difference to anyone. No vehicle should ever be parked within 10m of a junction (except, of course, in a marked parking bay). There's no PCN code for doing so but it renders the driver liable to prosecution for causing an obstruction. (Anyone who doesn't believe that should just watch the traffic wardens in Clacton on a hot summer's day. 'Booking' cars parked near junctions, despite the absence of yellow lines, seems to be a favourite activity of theirs!).
Chris