I lived several years in London before having children, and have raised my children in several European countries (Spain, Italy, Luxembourg, Switzerland) and we are now in Australia.
My children haven't been enrolled in the Uk educational system but I believe the Australian one is very similar to the UK one, so it might not apply to UK.
With children, I would go to the South of France.
I am horrified by the Australian education system, the stress and obsession of private or religious schools, the shallowness of the content and the huge gaps it leaves. I have 3 children, my youngest arrived here in Year 1 after having had a single term of Italian primary school in which she learned to read (started school in September not able to read like most of her class, by December could read everything , like most of her class, typical syllabic method, B+A=BA) , she joined a Y1 class here in January, in which half the class struggled to read in the phonics system, and this struggle continued for many up to year 3. My DD didn't know a single word in English, but could read out of an English book with the worst pronunciation and accent until she learned the language and then her reading sounded English. My eldest is finishing Y12 this year, so I have now seen all the content of primary school and all the content of high school. OMG the gaps in education, shocking, especially in history and literature, what we would call culture générale. In France, we do a century/year, so the likes of Rabelais and Montesquieu one year, Rousseau and Voltaire the next, and so on up to the modern authors Camus, Sartre in your final year. Here you get Shakespeare every single year, it is as if they only had a single playwright you do a different play every year.
The same in history. Massive jumps and gaps, no chronological order of events when in reality every event is the consequence of the event prior to it.
Geography doesn't include the world, its nations, main rivers and mountains, but focuses on globalisation, agriculture, which is fair enough, but then people have no idea where some countries are, including some that are the closest to Australia's border. A mother didn't know where New Caledonia was, when it is the closest foreign country to its borders.
No ideas on painters, musicians, there was a Monet and impressionists exhibition and I asked a friend if she wanted to come with me, she didn't know who Monet was or any of the other painters.
Luckily as a family, we have tons of books and travel to Europe twice a year so take the kids to Museums and art cities.
The level of culture is extremely low.
I would recommend you find some French expat forum in UK and ask some views about the content of school curriculum.
In Uk , the medical system will be very different. GP and not paediatrician care for children, if you need a specialist visit or test, it is usually very hard to get one in some part of UK. This is something I have really missed here. Children are not small adults and GP can't really do and know everything, especially when it comes to mental wellbeing.
Another thing that has surprised me here is how body-shy people are. And I live on the Northern Beaches near Manly. Very body conscious, quite ashamed, lots of complexes, in French, we would say "coincés et mal dans leur peau".
We have been here long enough to become Australians, but I miss Europe, the lifestyle, the food, the happiness, the laughter, la douceur de vivre.
I had fun in London in my 20s-30s, not sure how fun it would be with children. Maybe Cambridge or Oxford, both remind me of good old Europe.
In France, you would also have the advantage of being near the sea and near the ski, weather is nice, and beware of the racism.
I have friends who work as lecturers at Oxford University and since Brexit, they have received abuse in the street or the bus when talking French to their kids, and being told to get back where they came from. Not once, not twice, several times.
Australia is beautiful but what makes you happy in a city or country is not how beautiful the country is but the people and the friends you can make. Maybe I came to Australia too late in life, but I struggle here, after having lived in Italy and Spain. To be fair to Australia, Switzerland was a lot worse on the friendship front (and racism!!) .
Don't go to Switzerland!