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Only to a radial two gang socket, Yes. That way, at least I know that it is going to take the same sort of load as the rest of the ring, which is literally wired with 5mm cable. I don't pull teeth on the extra 1mm strand. For a single gang radial socket, then I'd use 2.5.
Thinking of a ring that way will give you a very misleading impression of how it works.
And for longer lengths, you're likely to find the impedance is too high to meet the trip times.
If the short happens in the connections to the back of the socket... ?
This is in the context of the two gang socket being unswitched and being remotely isolated by DP switch from the ring itself.
I think the difficult thing with a reduced size cable on the spur is coping with a wiring fault (or damage) to the spur itself.
The fuse in the plug will protect the flex to the appliance, and in the case of a fault in the appliance it will also protect the spur wiring. However a short in the socket itself for example, would necessitate the spur wire being able to take enough fault current to blow the main fuse on the ring circuit.
So basically if you are confident the reduced size cable will perform well enough under fault conditions then you can use it. I suppose the argument for "skimping" is strongest when you have a contract to wire five hundred flats all to the same wiring design - the saving in cable costs may then be noticeable. From a DIY point of view its a wast of time IMHO.
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