No electrical wire or connection should be plastered over.

Eh?

Could someone please provide a link to a suitable joining piece (either
crimp or solder).

Put it in a conduit, either metal or plastic, so it can be withdrawn.

Nice idea, if you have the depth to do so. Not required by the regs in most
case.

My screwfix catalogue seems to only show standard crimp connections.

Then you should avoid Screwfix like the plague.

Why?

Christian.