So my instinct was to say that you're being difficult and shared parental responsibility means what dad does with the kid is in his care is dad's issue. BUT you have sole parental responsibility... Legally I don't think dad will have much chance in court if he pursues this...
Dad certainly could have gone about this better - he could have notified you prior to booking the tickets and provided you with a JP signed copy of the 3rd parties passport... Then the courts would have possibly had grounds to be satisfied that you're being difficult and not him...
Just outa interest - and i doubt you'll write your answer... Is your decision to refuse travel based on genuine fear for the child? Do you actually fear that this 3rd party is gonna sell your kid into slavery of something? See my thinking is it is best to do everything possible to make things work. At least that way if this goes to court you seem reasonable.
If you fear that this person is gonna harm the child while on a flight. Let's think this through, you drop kid off at airport, kid is in a very safe environment from then on.... Lots of security, etc, etc at airports... So really you have nothing to fear...
Next - what have you done? Have you asked dad for a photocopy of person's passport? Is the same person flying back with the child? If you were provided with the relevant details of 3rd person would that appease you? It would at least make you look better in court...
So if this goes to court you want to prove that you did all you could to get dad to appease your concerns. It is his choice if he plays along...
On a strict reading of the orders, I'm assuming it doesn't say that the dad must be present during travel? So on a strict reading of the orders I think you're in the wrong.... And in your last post you say you were not given sufficient notice. So if dad told you 3 months ago that would make a difference? I fail to see how.
It would be the same person on the same flight, the only difference? You're given more notice... what does that matter?
My opinion, do all you can to get suitable documentation from dad... Realise, no court will punish you (IMO) if you don't send the child - why? Well clearly there is some history here that I don't know about, sole parental responsibility hints at something...
Look, we all worry about our kids, my kids are currently with their mum and frankly that scares the **** outta me because mum has some bad history... But the courts ordered time with mum, so the kids have time with mum. Like I said, this one is entirely your choice...
So all things being equal - I would send the kid. The kid isn't gonna get damaged by sitting on a plane with someone they don't know. And you're stopping dad seeing the kid because of a choice to interpret the orders a particular way and because the orders do not specifically say dad must attend flights, I think you're in the wrong. However, I understand there would be a few other practical considerations...
Does this person speak English? Does your child speak a common language ?
So to summarize...Can you stop the kid going? Yep. Should you? I don't think so... Not if your only fear is that you don't know the person the kid is sitting beside on a plane... Try and resolve it by asking dad for specific info about the third party... That way at least if it does wind up in court, you can show that you had some concerns, tried to work with dad to sort them and he refused, hence he is the problem, not you.