Apart from stating the obvious (Central London) where big financial is regularly advertising for Linux, the two corridors stand out.
Between Warrington and Leeds the M62. With a particular focus on Manchester.
The M4 corridor, not that it is news to see this area mentioned for startups.
Cardiff and Bristol - a nice tech cluster going on there either side of the River Severn.
Developments in 2012 and 2013:
The Silicon roundabout and other initiatives should help Linux move further east of Central London. At the moment skills are concentrated in Central London and other parts of London, but not much in East London.
( Middlesex and/or the stretch Left of Central London between M1 and M4, has always traditionally been a tech hotspot also )
The Silicon roundabout will stretch the reach of Linux a little further along that southern corridor, but what about the Midlands and North?
Tech startup success in Warrington, Manchester, and Leeds should spur on the extremes (Liverpool and Hull) to eventually play catch up.
Birmingham is underrepresented, and I will keep watching the news for any development there.
Linux and Tech Startups:
Take the map above and overlay it onto a map of startup clusters. What do you see?
Note: That map at the top is a point in time analysis, and will change month to month, however I suspect the picture will look broadly the same until those initiatives mentioned above take off.