Why has some of the English Regions switched to a new theme? I believe that there is now 3(?) English themes out there now?
Choice of the editors. BBC South East decided they wanted to break away from Corporate in December 2009, using a higher pitched version of the title music specially commissioned by them and written by David Lowe. Midlands Today then followed in March 2010 to coincide with their new set. It stayed that way for months, then North West Tonight/Today used the same music as their special event music (Christmas) before taking it up full time when they moved from Oxford Road.
BBC London used the original title music for the 2008 relaunch for 2 days at the start of this new look from the 21st April 2008 - 23rd. It was then dropped, Midlands Today started using it again (and a mix of the SET/MT/NWT music stings and closes mentioned above) in June this year
You are right, there are 3 different pieces of title music in use across the regions, the most used one being the original from 2008, then 2 (and a half?) regions use the SET/MT mix from 2009 and Midlands Today, being themselves use a hash of everything but predominantly use the pitch music from 2008 that was used 2/3 times maybe!
I won't mention when MT slowed their old 'People and Places' titles down and mixed the 2007 nationals music to it for a week!