Help with specifying cable

[Dennis82]

hi, I have a small consumer unit in a room that serves a 1.5mm2 light (18w), 2 13A double sockets and 1 single socket with a radiator (1000w). This consumer unit is connected to the main consumer unit. Can I ask what size cable should I use given the loading in the room is small. Can I use a 2.5mm2 twin core or does it has to be 4mm2 or 6mm2?

Also if I want to connect a sub meter to record usage, should that cable connect to the submeter and use the same size cable to connect from sub meter to the main consumer board?

Thanks.