Each circuit has an RCD type fuse in it Lights, cooker, smoke alarms, plugs and I think the shower as well. Using a separate / stand alone RCD on the plug circuit trips the switch/fuse when its tested. My Mother had the old fuse wire type fuses replaced with the RCD type ten plus years ago.
Well worth having it done in my opinion and this comes from someone who has had shock off someone else's shoddy work. My Dad used to regularly come across shoddy electrical work whilst he was a heating engineer and I know hes had a few shocks from unsecured wires!