Plugs are fused because the current capacity of the in-wall wiring (and the breakers that protect it) is much higher than the safe current capacity of a single appliance's power lead. The circuits are often 32A and most appliances are rated for 13A or less. So without a plug fuse a fault in the appliance that draws, say, 25W would potentially overheat and set fire to the appliance cord without tripping the breaker. This is true of most countries' larger radial circuits too, I think, and the British practice of fusing plugs is a safety upgrade. How much risk it reduces in practice, I don't know.
The final ring circuit is an idiosyncracy of British wiring practice, where the cable goes out in a loop from the distribution board to sockets etc and then connects back to the board again. It allows smaller gauge wires to be run for a given current rating, and was introduced after WWII to reduce copper use during reconstruction. This does have some unusual failure modes, but the code is absolutely fine for this system as long as it's followed (including the testing regime after new installations, which will catch, for example, a ring circuit with the neutral conductor disconnected only on one side of the ring). Obviously bad work can happen anywhere.
