That is disturbing... What is colloquially known as common sense should dictate that live exposed conductive parts should be covered never mind BS7671...
Burying T&E??? common...
and are the fittings even IP rated?
That said, please excuse my naivety but what is the problem with having the lights fed from a ring main (assuming the additional load would not take the circuit beyond max rated demand).
I have just done something similar to a new build house where they had run 6mm to a hob point that was going to feed an induction hob, but instead the owners fitted a gas hob and so there is 6mm cable on a 40am MCB to feed a tiny electrical spark... I have added a circuit that will feed a couple of electric heaters, a few lights and sockets.
I am a qualified electrician, but not a very practiced one (just qualified). and so am checking that I have not made an error that will have my work appearing on a new thread...