Extending cables to a consumer unit?,

Usual rules for junction boxes apply - so as long as you have a secure join, and adequate insulation and enclosure, then arguable anything within reason goes.

For elegance and order, I would suggest a small metal clad box with din rail inside, and a row of din rail mounted terminals. You can get Wago style ones, which make for quick and maintenance free joins.

You can also get (or could 3D print) clips for mounting sets of 3 wago

e.g.

Alternatively, can you get under the floor near the CU to use a Wagobox (or 2) to extend the cables?

I don't think I'd be tempted to print my own wago holders, rather use commercial ones that will have proper fire rating ...

Well if you can 3D print ones out of the same material, like ABS, its isn't an issue unless you have some regulatory authority breathing down your neck.

It is, as usual the difference between being fireproof, and having a bit of paper that says so (even if it lies)...

AS an aside once you have a 3D printer and have learnt to design in 3D its amazing what you can do. I got sent some RAACO cabinets without drawer dividers. Hardly the most complex thing to design and way cheaper than ordering the correct parts. OK they are not transparent polystyrene, but who cares?

I made containers for the daily pills, and tools and so on. These are again simple designs. The cost for the machine was steep. But the material is fairly cheap...

Thanks for that John. I think a metal enclose and DIN rail, got a lot of that around, and some WAGO connectors and sorted:-)

Haven't got one of they new fangled printing presses as yet!..

Nope, this is around 5 foot up on a wall!..

Bit of an odd arrangement not a wooden floor but concrete..

The fire rating mostly comes from the metal box.