You've hit 2 nails firmly on the head there.
1) Good/Bad practice and the instant LABC expert. This is really a distillation of the environment, encouraged by the OSG, of what I term "electrical installations for the hard of thinking". The reliance on the use of standard circuits means that people don't bother to do proper design, or think about what design principles underpin those standard circuits, they just follow "the rules" and don't consider other options or other designs.
2) What the "spur" feeds. You're right about the cooker hood load, but your tumble drier comment is interesting - I'll bet there's at least 1 person in the country with a T/D plugged into the integral socket on a CCU not knowing that the design current for that socket is only 5A. We're back to the "rules", and how you either have to unthinkingly follow them or work things out properly. A spur in 2.5mm² is considered safe because the rules say only 1 socket, so when you follow the rules you can't overload the cable.
When I said "discuss", it wasn't a rhetorical, or sarcastic remark. A cooker circuit is of course a radial, so a circuit coming off of it is not a spur, it is a leg of the radial, and there are no rules explicitly limiting the number of sockets on it - you're into the realm of designing the circuit properly. There is no simple "only 1 socket" rule that "prevents" more from being added, which is why you do have to be very careful, and which is why I agree that you should not attach a 2.5mm² leg to a 32A 6mm² radial, except via an FCU to provide proper protection of the cable. A spur on a ring is a special exemption to the Ib <= In <= Iz rule, and it does not apply to other circuits.
