When my children were a little bit younger my favourite shop for buying essentials (and a few 'just because I like them') kind of things was Blue Daisy. My more local shop is in South End Green, Hampstead so perfect to have such a well thought out shop to drop into as you also manage to fit a bit of exercise in some wanders around the heath. A more recently opened shop is in West Hampstead.
My main reason for loving the shop is that I always felt it stocked a brilliantly edited selection of what you need for wee ones and I trusted if there was a particular item in the shop I didn't need to go elsewhere to search for other alternatives. As someone who gets a bit overwhelmed by too much choice this is my perfect way of shopping ... go visit if you can!