Usually at least 4mm2 for a 32A radial, and even then it depends on other factors.
OK, so, I will use 2.5mm2 "ring", and each room separately, so say 3-5 wall sockets, and fitted with a 20A fuse on the consumer unit?
But the kitchen : would that require beefier cabling as we have toasters, fast kettles, coffee makers, air fryers etc? Can I split the kitchen into two, so use 2.5mm2 still, but in two separately fused circuits? And have the W/M, D/W all separate?