The problem is that without one, if you do want to add another CU, or change one, there is no legal way to isolate the supply to it - your only legal option is the time consuming and costly method of waiting for the DNO to come out and pull the supply fuse.So I am getting the general idea that this additional isolator is not required!
Yes, I know that lots of people pull the fuse, and yes I know it's safe if you know what you are doing, and yes I know that the chances of being prosecuted if nothing goes t*ts-up are smaller than a small thing that is very small, but it is illegal, and nobody should be advocating illegal acts.
What did you read which made you think that?I thought it was an addition in the 17th edition that all board changes must have a isolator fitted!