Should a heavy duty transformer (used predominantly for LED lights) be on the mains ring or the lighting circuit?
Is this approach correct?


Basically, yes.

1mm2 twin and earth cable (I presume that is what you are proposing) could carry up to 16A, depending on where it runs, so 1.5A is well within the headroom (assuming that you aren't talking such long runs that voltage drop becomes an issue).

For lighting, however, I tend to use 1.5mm2 in preference to cable 1mm2.