OK Yorkie. I need to clear this up. Under the new(ish) 17th editions, it is now perfectly acceptable to loop on.......... because ............ the whole "chain" is protected by the initial 13A fuse.
In fact, it has been common practice for ever, to spur a ring main for e.g. a light fitting........... using a 5A fuse and 1mm cable!!!! The cable is protected by the 5A fuse.
I guess TOWIE is confusing this with a simple UNFUSED spur, which, MUST be in 2.5mm cable.
I remember that this area caused some confusion when I did the Domestic Installer course. The tutor made a point of saying that, it is now permitted to loop on as much as you like as long as it is covered at the "head" of the spur with a FUSE.