As DS has said, that would, IMO as well, be an unpleasant bodge. Having an in-line fuse outside of the back box would make it even worse - and, in any event, how could one then get access to that 'cavity around the box' to change the fuse? As I've said, apart from anything else, any arrangement like that could be a nightmare in the future for anyone who doesn't realise that there is a fuse hidden away somewhere.The backbox is 25mm but there is small cavity around the box that I could put the inline fuses. ... The solution above seems my best option.
There is no where that I would have anything like that in my house, or any house I had anything to do with!
Kind Regards, John
Sorry when I was referring to the solution above I meant with the GRID 20Amp switches ( as proposed by JohnW2 ) and the fused spurs on top of the units. I did not mean the fuse solution which I agree could be problematic.