Hmmm - Kensington really isn't the best place to commute to from anywhere is it?! Still if you think 20-30 mins from Kings Cross, plus bus/tube from there is ok, then lots of Hertfordshire places could be contenders.
If you like the hustle and bustle of London, then St Albans may well be the best place for you, although £500k may still not buy you all that you would like. Property in good parts of the county, esp those with good commuter links, is expensive.
I would personally prefer Harpenden to StA, but I'm not really a big city girl, so that probably doesn't count for much! Harpenden does have a good atmosphere though, with lots of places to eat out, some nice shops (though not the big stores of StA) and the largest soft play centre in Herts to keep the kids happy!
You could also look a little further north, although we'd be talking villages here, which would take you to Luton Airport Parkway as your rail station (the one after Harpenden). The frequent trains from there take around 32 mins to Kings X, but there are also some Midland Mainline trains which stop here, and these will get you to London in 25 mins.
If you chose to go for Hitchin, your money would buy a lot more in property terms, but the journey is just over half an hour to London. Hitchin is a very family oriented market town, with cobbled streets, a mix of small independent shops and better known stores (Waterstones, Starbucks etc), but none of the big department stores.