Do similar junction boxes exist for use in the space above the ceiling?
The MK et al ones in my various catalougues don't seem to have quite
the right capacity. I tend to move the rose up there, but don't find
it a very elegant solution.

A standard round 4 way 20A terminal block is what you need. The terminals
are used for earth, neutral, switched live and perm live. They're not as
easy to wire up as the loop terminals, though, which have separate screws
for each wire, rather than a large shared one, where the other wire always
pops out when you are tightening.

Christian.