The release version has omitted all the 'buses. The BritBus.pak file is actually an incorrectly named city buildings file, and the 'buses are not included at all...
Edit: For those who want to fix the problem themselves, rename BritBus.pak to BritCitybuildings.pak, and download the 'buses from here (http://simutrans-germany.com/files/upload/BritBus090512.zip).
Looks like something really wierd has happened to the build process... it definitely all worked when I tested it!
It's almost as if the city buildings were built with the name for the 'buses, overwriting the 'buses.
As you suspected, James - will be fixed in the next release.