In theory a 2.5mm ring main doubles the area of wire, to 5mm, as one is actually running 2 cables to each outlet socket. The 13A fuse is discrete to each appliance.
And what if a there are 3 appliances pulling 10 amps each on one ring circuit, and a loose connection on one side of the ring burns open?
I (in USA) thought up ring circuits on my own back in the late 1960's in my teen age. But even then I could understand the above failure mode and suggested the idea in the form of completing each end of the ring into separate 15A breakers. The idea wasn't for saving panel space; it was for saving wire. This was back when there was a push to Al wire from Cu. Fortunately, my parent's new house was wired Cu, anyway.
