As EFLI has said, it would not be in a 'safe zone' even if it were directly below the light switch, since it isn't connected to that switch.[That's not as daft/pedantic as it may sound - it's not impossible that someone might one day remove the light switch {e.g. move it to somewhere else}, leaving nothing to indicate thatthere was a buried cable.
I know a lot of people say this, but where exactly in the regulations does it say that you can't "borrow" a safe zone? I read through the 17th edition and could not find anything saying that. The safe zones are simply defined in terms of horizontal and vertical areas from outlets. I can see why people assume that is the case but the wording of the regulations does not actually say that as far as I can tell.
Sure someone could move the fitting later on, but if they choose to do that then it is their responsibility at that point in time to sort out what would now not be in a safe zone. There is no requirement in the regulations to ensure that a safe zone remains a safe zone forever into the future.
Just to illustrate that point one could put something in the corner between two walls, which would be one way to traverse a floor level in a safe zone. However what if someone then remodels and removes the wall? Now the cable is not in a safe zone. If we follow your logic we can't use a corner because someone at some point in the future might remove the corner removing the safe zone.
IMHO running through a back box is better than a corner because you cannot remove the fitting without realizing you are losing a safe zone.