Pink Validators are my biggest annoyance. The official line is that you can’t start or end a journey using one. The reality is that if you touch one and you aren’t in the system then it will start a journey at that point. There is absolutely no way that a pink validator can end a journey though.
What I will say is that there is a yellow validator on platforms 9/10 at Stratford near the London end.
PS. Your favourite topic has recently resurfaced in respect of Kennington and I may just be about to document it.