Thanks for all the replies. It's been a busy month so I'm only catching up now.
Sadly, I think we probably have missed our chance for a number of reasons. I think the upheaval for our two children might be too much. One has been diagnosed with autism and the other is probably autistic too. They're pretty 'high functioning' but change is hard, especially for the older one, who has finally settled into secondary in a very positive way after a turbulent start. We're only beginning to understand what to fight for in terms of support at school and having to start over in a new country with a new language feels daunting, even though both kids have shown an interest in languages and DP and I are always keen to learn a new one.
The point @OrderOfTheKookaburra made, about needing to be in the UK for three years before uni to qualify for local uni fees also gave me pause. Thanks for mentioning that. I'm not sure I want to risk 'trying' a place for a year or two and then, if it doesn't work out, coming back here and not being able to afford university. It might also be hard to get back into our current school given how over subscribed it is.
We have UK and EU passports, so that was never an issue. We were mainly considering countries with newcomer classes to help the kids learn the new language. We know families who have moved to the Netherlands and Norway and had good experiences, but one went when the kids were much younger. The other is doing it with a 13yo but he is a compliant child and doesn't have special needs.
Jobwise, DP is a self-employed consultant, so he'd probably be fine, but I think it would be hard for me to find something as good as I have now. It would possibly introduce a level of uncertainty that I don't have much appetite for right now, unfortunately.
I feel like life in the UK has really deteriorated in the past 8-10 years and I do regret not leaving, but things feel so complicated now and I'm nearly 50 and I think I probably just need to make peace with where we are in life at the moment and make the best of it here. A more local move (from suburb to town) might be possible in a year or two and I think that could make a big difference in how I'm feeling about life here.
It was really helpful to hear everyone's perspectives and think through it all. Thank you!