Look OP - I get it, you like where you live, you feel part of the community, you like that you have a relatively short commute and so get more time together as a family that's not 'wasted' on trains commuting in from further. It's all lovely, and right now, you can afford to buy a place big enough for your family.
The 'cost' to all this is that your housing is so much higher than it would be if you lived further out of London and commuted in for the same size property. Now, right now, that choice works for your family, from now on though, you will have to factor in the fact that benefits will not plug the shortfall if you/your DH loses their job.
You are in a position where you could move to a cheaper property outside of London and use the money you are saving in mortgage payments to commute in (this would mean of course if you did lose your job, you'd also lose the train fair cost and then would be able to live off the capped benefits). If this doesn't appeal to you, then you need to prepare for the downside of your choice - so save in order to afford a period out of work/take out insurance policies to cover the shortfall.
You are aware that it could be an issue for you, you have the opportunity to take action, it is a choice not to.