If a new installation - or works which have to be checked - then it
would make sense to do this before laying any covering. If a
*thorough* inspection of an existing installation, floorboards would
have to be lifted anyway. If just a test, the existence of a JB or
any cable connector wouldn't show up - unless faulty.

Fair enough for ring mains. But there are other circs in which the
existence of a JB will be manifest. Eg I have a hall light which is
two-way switched using a traditional junction box under the first floor
floorboards. (I wish now I'd re-done it without but there were time
constraints - plus the constraint that the light hangs via a 6x4 joist
and I did not fancy drilling a big enough hole through that to feed a
junction box.) AIUI any PIR which looks at that light will show the
existence of the junction box and expect it to be available for
inspection. So do I have a way now of avoiding the potential need to
lift the boards?

I had wondered about placing the junction box in a further enclosure
which I then fill with a pourable epoxy. (ISTR doing something of the
like with electronics for use underground many moons ago.) But that
still does no prove to the inspector that there were good and tight
connections before the resin was applied - or indeed that the resin is
really there (despite eg photos). The same seems to be true with
soldered joints. So am I basically stuffed (or whatever the correct
term is under BS 7671:2008) unless I can provide access?
--
R