Yes, from what I remember of the IEE 14th Edition, 1mm is for "lighting circuits" and 1.5 mm is for "heavy lighting". 1mm usually fused at 5 amps but will carry about 10, 1.5mm will carry 15 amps. 2.5mm can be used "singly" on a spur at 20amps but is normally used on a ring circuit and fused at 30.