Pintpot, I think you're absolutely right on the legality of any agreement and certainly wouldn't say it safeguards anything. That said, it is probably not a wasted exercise if it prompts parents to at least consider the potential repercussions of a move. Marriage breakdowns are of course not so easy to predict but if a relationship feels rocky, it would be prudent to think very carefully about moving away from support networks.
Some of the hard facts about what happens... aside from being stuck somewhere you may not wish to live til your children are adults... if, say, a family member in the UK is hospitalised, you can't rush to their side. You can't just take a holiday to visit loved ones unless your ex agrees to it. If your parents are elderly and unable to travel here, you can't just go and visit (unless you're OK with leaving your kids behind). You might not see them again. You need to consider before moving (whether to Australia or the UK) if you can accept these things. For some people the move can work out great, for some terrible - better to at least be prompted to consider the risks.
My mum won't be able to make the journey here again. I haven't broken that news to my child yet but had to wipe little one's tears this week because Grandma is missed so much. I hate that I put my child in this situation. So, forgive me if I am whining a bit here - sometimes venting helps me put back on my happy Mummy face before doing the kindy run. I wouldn't say to people not to move so much as to just be very aware of what they are taking on and avoid some of the shocks.