I'm a local financial adviser and I recommend Z Group for my clients looking for an accountant. They are based in Wimbledon on Arthur Road. Others here on NappyValley have also recommended them so in the past. http://www.zgrp.co.uk/
One thing I like is that they also like to meet their clients in person. (In fact I went to visit them today!) They're also very client-centric, not forcing you to work the way they do things, which I think is important - every client is different.
They're an established practice, friendly and approachable. They're good at tax efficiency, but have a broad range of services. They have a large number of self assessment clients, but also work with business owners.
(FYI, I'm not affiliated with them in any way other than I rate what they do and how they do it! Always happy to recommend good professionals.)