dennis@home explained on 17/10/2019 :
UK sockets aren't designed to have the ends folded over.
Folding the single core enough to get it in will probably result in stress
fractures. Then you have to be sure the screw is clamping the unbent bit and
not the bit you have bent over as it could break later due to thermal creep
and vibration.

Folding works fine, where there is only one wire to go in the terminal
and the hole is too large. Idea is to fill up the hole, to promote best
contact between wire, terminal and screw.