I was going to suggest Clapham!
I have the opportunity to work from home next year, so we realized how much more we could save by living in the city compared to Herts. With my husbands travel (to Putney) + what we spend on our 1 bed flat, we realized we could easily afford to live in London and be no worse off than we are currently. Most of our weekends are spent in London, so we are probably spending more money than we would in London.
We are looking at £200-300/pw as that's what we are currently spending. In Putney & Clapham it is totally possible to get a 2 bedroom for that.
I am not sure about Harpenden, but Hitchin has really expensive council tax, for a tiny 1 bed we pay DOUBLE the price for a nice 2 bed in Putney, which makes living in Wandsworth area extremely attractive.
For us, moving to the city is the right choice, as I am not really happy in Hitchin, my DH isn't happy with the commute (esp. last night when the train broke down and it took him 2 hrs to get home from Kings Cross) and we aren't saving any money (or having a bigger place) by living outside of it.