Hi OP I haven't read all the pp but want to give you our story for reference as we were in a very similar situation. We were in a flat in zone 2 nw London quite a few years ago. Dh was working for a company based in London and a city quite near Derbyshire actually. We decided to move up north as we thought it would give us a better quality of life. Only two hours on the train and we can come back all the time we thought.
In our new city we lived in a "naice" neighbourhood. Beautiful house with extension/loft. Good local school etc. Lots of disposable income so comfortable lifestyle. Seemingly "perfect life" but after the novelty worn off I became quite unhappy. Countryside was nice but I was bored with the limited options of activities when it's raining and we couldn't be outdoors. As DCs got older there was increasingly lack of cultural events to take them to. And I missed lots of things in London I would've liked to go to. People were great and we had a lovely circle of friends but I felt like everyone was so similar and I miss the diversity of people I meet in London.
Also we realised that we both wanted something new in our career and the city we were in just didn't give us enough opportunities to grow.
Eventually dh found a job back in London so he started commuting again. A month after he started his job we decided to move back to London, after being away for four years.
The stupid thing we did was that we sold our zone 2 London flat in an area that was fast gentrifying when we moved. We were convinced we'd love the quieter lifestyle! The price has pretty much doubled since we sold it and we could no way afford our old area.
Instead we moved to a modest house in zone 3 north London. Not "naice" but lovely and community oriented. A massive step down in terms of housing, but we all LOVE it. The countryside is still easily accessible. We go to country parks almost every weekend, and lots of variety of things to do.
Been back 2+ years now and so thankful we made the decision to move back.
I think if you want to try moving up North, don't commit to it fully before you try. Rent, don't sell and buy like we did. That was the worst financial decision we've made in our life. If we held on to that flat and swapped for our current house we would've been nearly mortgage free!
Obviously your situation is probably different from ours but just thought you might be interested to hear our experience. I think there are thing you take for granted in London that you won't realise until you move away. Lots of places others have mentioned that you should try exploring before moving out completely!