In uk.d-i-y, Dave Jones wrote:

My understanding was that if you have a any cable which requires a join,
that join should be of equivalent rating to the cable. Other wise if Ross
sells up in 2 years, job bloggs moves in, sees 10mm in consumer unit,
upgrades the fuse and installs a 12kw shower, not knowing about the under
rated junction box in the loft.

While it's good belt-n-braces practice to follow the guideline you suggest,
it's not required: it's the responsibility of each "joe bloggs" making new
use of existing cabling to satisfy themselves as to its length, routing,
condition, and so on. It's not using a 45A-rated connector in a cable feeding
a non-overloading design load drawing 40A which is the negligent action:
it's seeing a 9k6 shower on the end of a 10mmsq cable and blithely sticking
on a 12k unit. No-one reusing the 10mmsq cable can know without inspection
whether it was put in at that size because (at one end of the possibilities,
meaning simply well overrated for the job) it's what the sparks had on the
van at the time, having just run fresh out of 6mmsq and not having time to
pop down the road to WF before the end of the day, or (at the other end of
the spectrum, where 6mmsq would've been well underspec and even 10mmsq is
only just OK) because 200mm of it runs through thermal insulation (through
the insulation in IMM's loft, maybe? ;-) and its capacity is therefore
reduced to 43A.

Stefek