I don't know what that refers to but you will use 2.5mm².
Twin and earth is not flex.
T&e has a bare earth wire.
Flex is flexible cable and has three cores the same
Ban All Sheds asked why 2.5mm .
I understand that about the twin and earth, but I can use the flex? https://www.edwardes.co.uk/en/produ...te-circular-pvc-flexible-cable-100-metre-coil
The flex is easier to handle.
If not I have a roll of this in the garage aswell. https://www.edwardes.co.uk/en/products/2-5mm-6242y-grey-twin---38--earth-cable-per-100mt-reel