We've jut submitted application for planning permission and so far are happy with our architect. Based in walthemstow but think they cover wider area. Let me know if you want the name.
I searched the RIBA for architects in my borough and the surrounding boroughs. Checked websites as that narrows down the list (either didn't like the look of the work or the site was very poor) and then asked for quotes. Had to discount a couple as they wanted to charge a fee just to meet. I work in a professional services role and would never pay someone to come and tell me if they can help, why I should use them and provide a rough quote.
We asked for references as well. So although not gone with someone recommended to us we were happy with choice.