Ring mains are protected traditionally by a 30amp or 32amp overload protection, this is more then a standard 2.5mm cable can carry but because it's in a ring, the current is divided down both paths meaning the protective device would see an overload and trip before any or the cable in the ring is overloaded. The issue though is if you introduce a spur, it is then possible to overload that stretch of cable given that it's been protected by a 32amp so to ensure this is minimised, regulations will have you fuse down when multiple points are present on the spur thus usually limiting it to the 13amp fuse or what ever appropriate size fuse you fit (nothing stopping you fitting a 16 or 20amp inline if the cable calcs are met on that particular spur but requires a bulky point for your mcb as oppose to a traditional cheap 13amp fused spur).
