If you are wiring the lights in 2.5 t&e then it shouldn't matter. It can cause a problem if someone comes along and isolates the lighting circuit to work on the switch and doesn't check that all the wiring is isolated. Of course if you isolate the lighting circuit and the lights don't go out you know it's on a different circuit.
I'm guessing you only have switch wires in the light switch back box otherwise you could take a live feed from the switch and feed it into your new circuit.
If you are wiring the lights in 2.5 t&e then it shouldn't matter. It can cause a problem if someone comes along and isolates the lighting circuit to work on the switch and doesn't check that all the wiring is isolated. Of course if you isolate the lighting circuit and the lights don't go out you know it's on a different circuit.
I'm guessing you only have switch wires in the light switch back box otherwise you could take a live feed from the switch and feed it into your new circuit.