Lighting outside?

Hi, I am going to fit a standard 230v light in my shed. I want to take the power from a socket and was going to fit a FCU with a built in RCD.

I know a lighting circuit is 1.5mm and the socket is 2.5mm what shall I wire the light in if I am taking it from the socket 2.5 or 1.5

Any help would be appreciated

I ask this because some guy who works with me is an electrician (newly qualified) he got confused over what to do and I doubt him.