Plugwash wrote
there is nothing wrong with mixing cable sizes in a cuircuit (and with very long runs you often have little choice because of volt drop and terminal size issues)
using cables with ratings below the current rating of the cuircuit is a different matter. Its generally considered best not to but provided the short cuircuit protection upstream is adequte and there is suitable overcurrent protection downstream it is ok.
look at your spurs from rings (protection from the plug fuse for the spur)
So, at the risk of flogging this one to death, there's nothing wrong in taking a 2.5mm2 line to a fcu from a 10mm2 cooker radial, because the 13A fuse in the fcu will pop, protecting both the line before the fcu and the line and appliance after the fcu?