BS 7671 is what I work off
Also known as the 17th edition wiring regs
it does not harm to go oversize on cables it is just unnecessary extra cost
bit like fishing for Minnows using tanker mooring lines realy
For god’s sake how much extra is a couple of meters of 2.5 over some 1.5? About 40 pence I would guess.
I’m well aware of what regs your working off, I’ve been using the regs since not long after the the start of the 15th edition, though house bashing isn’t my thing TBH as I’m more industrial 11Kv to 415v thats more my area.
Its just I’ve seen too many panel fires/flash-overs due to scrimping on cable sizes that these days I tend to spec higher.