I would assume the term accessible within our industry would refer to the common practices done to inspect and test a property, as a whole, ripping carpets up and pulling boards up wouldn't be carried out on such an inspection so any joint that wouldn't be expected to be checked, would in my mind be classed as inaccessible, I would expect any sparky worth his salt to stick his head in the loft and check for joints etc unless its boarded out then they should be MF.
This is my opinion and based on common sense of joints that are unlikely to be checked during a routine inspection report so any chances of obvious issues that could be spotted with a visual but not picked up by testing is reduced with the use of MF joints.
I'm not suggesting hiding JBs is good practice - merely stating that at least the plumbers have some proper guidance on what is considered "accessible" in their industry.
Regarding loft spaces, everyone I look in seems to have 18" of fibreglass insulation covering pipes, cables, downlights & LV transformers. I for one don't remove it all in the search for JBs that may or may not be there.
However, there must be literally millions of round JBs hidden in the UK domestic sector, with only a small percentage causing any problems. The test part of the I&T usually shows up any problems, then the fun begins.