"I wonder if that's because usually both partners work - this doubles the transport costs. "
Not necessarily. Two season tickets from Shenfield is £7,760, two from Brixton £2,500, the difference is £5,260.
£5,260 on a mortgage is around £132,000 at 4%.
So let's say you have £600k in Shenfield: www.rightmove.co.uk/property-for-sale/property-44494628.html
and then £750k in Brixton
www.rightmove.co.uk/property-for-sale/property-30811554.html
The Shenfield place is far too expensive obviously, and needs at least a third off the price, but the £750k to live in a tiny ex-LA-type terrace in Brixton is in another universe.
And if you are working in the City, the Shenfield train is 23 minutes straight to Bishopsgate, which is less time than it would take to get there from Brixton.
It's a mania and you can't begin to rationalise it. People are searching for the bigger idiot. Yes, if you want to get the London salaries you do need to live somewhere near London, and the £600k for the family house in Shenfield means you might well be better off oop north with a lower salary and a house at half the cost, but there's absolutely no need whatsoever to pay ££££££ to live in Zone 2 Stabsville.