Plastic or metal pipes? what do you recommend?
TLDR: copper pipe with compression fittings.

In the UK, your choices, in approximate order of decreasing compactness and decreasing skill required, include


Copper pipe with end-soldered connections.
Copper pipe with solder-ring connectors.
Copper pipe with compression fittings.
Copper pipe with push-fit connections.
Plastic pipe with push-fit speedfit connectors.
Plastic pipe with push-fit Hep2o connectors
Plastic pipe with push-fit Polyplumb connectors
Plastic pipe with push-fit Floplast connectors
Plastic pipe with other push-fit connectors.


However, if you already have copper pipe, I would use compression fittings and copper pipe (if you have space) to maintain earth continuity. You probably need to add equipotential bonding where the pipes join the taps.

Plastic pipe is easier but, in my experience, the couplings are more bulky than even compression fittings and this can make a significant difference to the end result (e.g. being able to fit joints within existing space).

Plastic pipe connectors usually have to match to plastic pipe type. Some plastic push-fit connectors (but not all) can be used to join copper pipe to plastic.

Plastic pipes can be bent, but I've had nearby push-fit joints leak, so it may be best not to rely on the flexibility too much.

The disadvantage of soldering is that you have to make sure the pipework is empty and dry, even a small amount of water several inches away can make it impossible to raise the temperature of the joint high enough for solder to flow.

So really it depends on how handy you are with a blowtorch.