I think gravitating away from the City is a natural thing for a lot of us. We work from home for the most part and from our mid thirties moved 4 times to places basically all scoped out as potential settling places, it took the fourth move to finally set long term (probably forever if I'm honest) roots down.
Depending on how much you want to slow down...Bristol is a good shout, Brighton is similar. We lived in Brighton for a bit, socially you are catered lite.rally for everything. We actually moved on BECAUSE of that...saving money was hard, so much just going on all the time. (Lewes is an option)
The market towns are almost always pretty pleasent. I suppose anywhere 'nice' might come with higher costs attached?.
Winchester, Salisbury, Guildford, Windsor, Kingston if heading SW.
The A25 has some lovely towns and villages along it. Near to the M25, train links to London and easy to Gatwick. Reigate was one of our favourite places we lived.
Cambridge is a lovely city and good access into the Lane if you go to games?
I love being a 'visitor' to London these days, a short blast of the bits we love to.
I'm sure others will give you some pointers.
Reigate is nice but isn't far off London prices now, in the last few years so many people from London have moved there that it's just one big building site - everyone is putting massive extensions on etc.
Horsham is quite nice, decent transport links. Looking at moving round that area myself in a few years.