I'm also messing around in my shed, and that's the way mine is set up, ie.
2-way garage unit, 16A and 6A. From there, 1.5 to a junction box, then
straight into a fluorescent. In other words, I'm using the fuse as a light
switch, which is plain silly. To fit a switch, do I have to come off the
junction box, or is it permissible to just interrupt the feed and run the
light through the switch ?