It`s only a "little known law" because people have either forgotten their Highway Code or didn`t read it properly. In the early 70`s, green cover H/Codes (6p & 12p) Rule 74: one has been allowed to stop in the box waiting to turn right if your exit road or lane is clear,& you are waiting for oncoming traffic. However, this caused problems at some junctions because only one vehicle would get round each change of the lights, because the following driver regarded the 1st vehicle as blocking their exit from the box. So, in the 1978 (black & white cover, 25p) version, Rule 92 adds the words "or by vehicles waiting to make a right turn".
So, dear Dollie your beloved offspring is wrong (as are many other drivers one sees confused about this Rule.)
If a lot of vehicles turning left into your desired road blocked it for you while you were waiting, you haven`t committed an offence as long as your exit was clear WHEN you entered the box.