Listenup: I think you're confused. Would you really think that gear manufacturers would be putting one spec on their equipment and in the manual, but then agreeing to something else in practice on the QT without announcing it, even though it affects the operation of their equipment, as well as warranties? This discussion is first of all about the manufacturers of fuses. I'm sure most gear manufacturers by now are well aware of what's going on with aftermarket fuses -- I believe Paul McGowan uses SR Purples in PS Audio's listening room equipment. However, they aren't going to get involved officially for obvious reasons. If you have a question about the appropriateness of going up a value, then contact the gear manufacturer to find how much tolerance they are allowing. However, don't expect any encouragement re aftermarket fuses, to say the least.
I don't know about mains outlets being fused, except at the residence's fuse box (U.S.), unless the owner has a separate electrical set up for their system. Not being in the UK, I've also never heard that the main purpose of fusing an outlet is to protect the cable, as opposed to the gear the cable is plugged into (from my Furutech outlet, the power cord runs to a PS Audio Regenerator, which is fused currently with a 6.3A SR Purple, vs. 5A spec). In the U.S., the biggest risk is lightening strikes, which would presumably blow a slightly higher amperage fuse just as fast.