Is that the law, the Law Society rules or just good practice?
Law society rules
If a parent gifts a house to a child, the child must get separate legal advice?
Yes
Seems odd to me.
It deals with the question of (possible) undue influence
In the current case, I think it's unwise for the son to act as solicitor for the parents. If there is any falling out at a later stage, it could be very awkward. If they use an independent solicitor, the likelihood of a dispute is much lower.
True - but there is no legal reason why they can't act.
mf