I'm putting in a built-in bookcase into the sitting room alcove and there's a 2 gang socket where the side of the bookcase will be. There'll be a 30mm gap between the bookcase and the wall and this will be covered up at the front I want to drill a small hole in the side of the bookcase and use a rewirable extension lead for power supply. The socket in the wall won't be accessible so I want to put another fuse on the extension lead where it is accessible. Apart from daisy chaining two extension leads, is there a way to add another fuse onto the extension lead where it can be easily accessed? I was thinking of something the size of a table lamp switch but cant find anything like it.
No, you can't do that, inline fuse holders are used on low voltage systems such as in cars, not on the mains supply. It would serve no purpose anyway, as you will still have a 13A fuse in the plug, and a fuse in whatever plug is plugged in the other end, so you would just introduce another fuse. Often if a plug is going to be difficult to get at, the most obvious would be in stage lighting, or behind shop fittings, we can use different plugs and sockets that don't feature a fuse, however this cannot just replace an existing socket, a fused spur would have to feed it from a accessible location. This is most often seen commercially, not in domestic settings, and the set up would need to be designed by someone trained and competent. If your extension is just going to feed a TV, lamp, etc, then the 13A fuse in the plug is very unlikely to blow - my extension behind the TV has been going strong for 20 years or more!!
Extension leads with a fuse are also available: https://www.screwfix.com/p/masterplug-13a-4-gang-fused-rewireable-fused-socket-with-neon-white/90230
As @Jimbo shows you can get extensions with a fuse in extension, I fit 7 or 10 amp fuses so should blow before the one in the plug, however fused plugs should be in free air, and clearly when behind furniture not in free air, so not sure which would rupture first, this house I would need to remove display cabinet to renew fuse, last house I cut holes in back of book case so I could reach plugs.
Bulgin make 230 volt in line fuse holders, seen plenty used on machines, not suitable for domestic though