Reading around, the use of 1mm cable seems to be frowned upon and 1.5mm is recommended. I've pulled out the old light circuits and I've got loads of sheathed 1mm that I'm tempted to reuse...

For my new light circuits, 1 is downstairs with 14 lights and 1 upstairs with 10 lights. The loops would run no more than 15 metres.

I know mixing cable isn't also recommended... but if I use 1mm for the switch drops and 1.5mm in the loop, can I go with a 10A MCB for each circuit ?

Thanks again.

Yup, 1mm on a 10A breaker will be fine

The main reason that 1mm is avoided is that of durability, it's not particularly robust when being hauled around conduit and attics. If you're using second hand cable do an insulation and continuity test immediately after you pull it to verify it's not been damaged.