Advice on a simple loft light

Have easy access to both the circuit for the upstairs lights (can see where it enters the attic space, know a lot of it's routing) AND have a single fluorescent tube then ends in a normal UK wall socket plug. If I run an extension up the attic, I can plug in that light and get light.

I'd like to have a simple switch, it can go anywhere sensible in the attic space (there's a free standing brick chimney stack if that would be suitable) that would power the light rather than having to mess about with the the extension all the time.

What's the simplest regs compliant way to fit a switch, I'd assume from the 2nd floor lighting circuit would be the easiest so would need a (fused?) spur from that, I'd assume? Would I still need a sparky to eyeball it and sign it off? Is there a middle ground thing that could be as permanent as a switch while not needing to get a chap in to nod it?