In article .com,
"Staffbull" writes:

Andrew Gabriel wrote:
In article .com,
"dcbwhaley" writes:

No. Depends on the MCB/Fuse. If it is a 10amp then 1mm^2 is not fine.
1mm^2 should be protected by a 6 amp MCB.

It's not that simple. On a 10A MCB, a run of up to 22m of
cable is permitted in the OSG. (1mm^2 can be rated at up to 16A,
depending on circumstances.) In commercial premises where 10A
lighting circuits are more common, the length of cable runs
often exceeds those permitted for 1.0 and 1.5mm^2 cable, which is
why larger cables are often found on such 10A lighting circuits,
but such long lengths would be very unlikely to be an issue in
domestic wiring.

Hi Andrew, the total cable length including switch cables will be
around 15m

To answer your original question, 1mm is fine.
Even if you had a 10A MCB on the lighting circuit
(which is unlikely in a home) 1mm would still be fine.

--
Andrew Gabriel