lighting circuits off ring mains?

I would be grateful for advice on an issue regarding running a lighting circuit from a fused switched spur unit.
I had an existing outdoor 13 amp socket on an outside wall which was a paved patio area.
I have this year built a garden room where this patio existed.
My question is can I run a lighting circuit consisting of four down lighters ( each one powering 2 x 20watt bulbs)controlled by a two way dimmer switch from a fused spur unit run from the supply where the 13amp socket was previously?
I was always told mains and lighting circuits were never to be joined, although I know since my day these regs have changed. I live in a cottage where running cables back to the consumer unit (four rooms and solid walls away) would involve digging up floors to channel cables.
Any help ,advice and alternatives would be gratefully accepted. Thank you.