The job is now done. In the end I did use the Toolstation lever connectors:
the positive reviews were at least as numerous (if not more so) than the
negatives, and the spring closure seemed perfectly OK to me: with all my
effort I couldn't pull out the wires, and it's hard to imagine them being
subjected to a greater force, given that the cables are tethered and the
whole thing is behind a blanking plate. I don't know about the rating, but
I suppose time will tell.

I was also able to get a proper horizontal run, and marked the blue wires
with red insulation tape (not having any brown).

Many thanks to all for the help and advice.