Cable clips

In a loft, the wiring will usually be over the timbers and within a floor/ceiling it will be through notches or holes. Only runs along the length of timbers will need clips - even then, if it goes through holes before and after, then even runs along will be unlikely to sag significantly in a fire - which is the reason for metal clips.

In both cases clips should be metal. Wiring under the ground floor (assuming no cellar) or up the walls, backs of cupboards, etc., will be fine with plastic.

Plastic melts. You don't want live wires draped down if there is a fire. Brian

I believe you can use a mixture of plastic and metal clips by interspersing metal clips in a run of plastic clips thus limiting the amount of sag. The metal ones can be as much as 5x as expensive and I find they do not hold the cable as neatly as dedicated plastic versions mainly because the ones currently on the market are designed to clip a range of cables as a result they do not particularly grip any cables well and getting nice neat straight cable runs is difficult.

Tricky Dicky pretended :

Before there were plastic clips, they were all thin copper 'buckle' clips. They were as neat, maybe neater than plastic if care was taken.

You might plaster over them when clipped to a wall!

Why is it (relatively) hard to find metal cable clips?

Plastic ones are often just nicer to use.