As we are all using led lamps / fittings now it seems this should be a standard circuit.
3A MCB with 1.0/1.0 cable
Lighting circuits used to be based on 100w incandescent lamp / room at 1800 lumens which is equiv to 20w led.
What are the figures for max distance for a 3A type B using 1.0/1.0 as an addition in table 7.1 page 66 in OSG?