I'm extending a lighting circuit to add a couple of wall lights on the
1st floor. I've got the cables laid from the loft down to the back
boxes for the switches and outlets for the lights, and now I need to
connect the new circuit to the existing lighting circuit.

Is it acceptable to spur off the existing lighting circuit at the most
convenient point, or are there rules to follow when deciding where to
spur from?

Most of the diagrams that explain how to add a lighting circuit show
it coming from the last junction box / ceiling rose in the circuit. I
don't know where the end of my lighting circuit is, and sods law would
ensure that it's a long way away from where I want it. I don't want
to be drilling or notching joists if I can avoid it.

My plan is to cut a permanent live cable close to where the new lights
will be and install a junction box at that point to connect the spur.
I'm not sure of the exact topology of the wiring in the loft, but I'm
hoping it doesn't matter.