I'm looking again at the original question.
why would you want to add another fused connection after the first?
so:
ring socket <---xFCUx---xSKTx---xSKTx
if those sockets take more than the first 13A fuse they will stop (fuse blows).
so:
ring socket <---xFCUx---xSKTx---xSKTx--xFCUx---xSKTx
makes no sense - the same would happen. The first FCU protects the connected sockets.
That said. You need to look at what loads you are expecting to connect and where they are positioned on the ring. By design a leg on the ring (the cable between sockets) the Iz shouldn't go above 20A. If you are adding these sockets to either end of the ring, those closest to the CU - you could well do that. You need to calculate the leg length impedances for the final ring and calculate the expected current going both ways (parallel circuit). Someone qualified and with the correct test equipment can calculate these for you to make sure this would comply.