Yes, this is the best I've found as well, but it would be really nice to have a map with the locations of all the major firms in London
perhaps it's a project for my next holidays haha
I have noticed that quite a lot of firms are moving to Liverpool Street. Freshfields and Baker McKenzie will both be based in Bishopsgate, which is the current home to Latham & Watkins and Mayer Brown.
Then down the road you have HSF on Primrose street. Right opposite that is Broadgate Tower where you will find Reed Smith and Hill Dickinson. If you go down the road or two, you will find Pinsent Masons and Watson Farley & Williams. A bit further towards Spitalfields you will find Allen & Overy and Ashurst. Then, if you go even further out near Bank there is White & Case.
I do not think there is much firms around the Chancery Lane / Holborn area anymore, all I know is Macfarlanes, Mishcon de Reya, Hogan Lovells and Bird & Bird are located there.