We moved out about ten years ago, and my colleague at work moved out about six years ago. My ILs (Londoners born and bred) are about to do the same.
We moved back to my semi-rural home village and, yes, I missed the buzz and excitement of London for a few years but then another part of my personality came forward and I am much happier where I am now. I need quiet and space and plants and a good dark mild these days. DH never missed London, despite having being born and brought up there; he hates the place and thinks it an infernal city (as does my fil, who was brought up in the 1950s in Deptford).
For my colleague, the issue was secondary schooling in London. They just didn't want their daughter to go to any of the local secondaries at the time, and they could not afford private.
I think the key to success for us was that we moved out to a place that has a very distinct personality and where I had very strong ties. This helped my DH as it is pretty hard not to become part of the local fabric when most of the chaps in the pub over 65 know who you are, what you do, and buy you a pint to ask some random question about something they think is connected to your job -- all because they know your father-in-law.
What might help is thinking about the point when you really will not want to live in London. London is not a great place to be elderly, so you can take an age point and then work back, if you see what I mean. So if you think that you won't want to live in London at 75, then consider what age you will need to move out in order to be healthy enough to deal with an enormous house move and to settle into your new area -- that could be sometime in your late 60s.
Then take the time from now to your late 60s, think about the life points that will fall into those years (children going to primary school, secondary school, university possibly, the nature of your jobs, how your industry looks to be going) and think about what might be viable.
There is no reason why you can't move back to London for a decade and then leave again.
Also ... £600k will get you a very nice 4-bed semi with a garden in some "secret" areas of South London. Just do your research and travel round a bit, looking at roads on the edge of nice areas that fall into less desirable postcodes by a few metres. The difference can sometimes be a few hundred grand.