I have made a British English thesaurus for openoffice.org. I started with the Moby thesaurus and changed a lot of words (Using the spellchecker) to their British spelling, and removed a lot of words that were not in the spell checker (Some American, and some that are not in common use). I think it could still use some work, but I would like people to download and comment on it.
It is at http://brit-thesaurus.sourceforge.net. You will probably want the -bin file if you just want to install it and go (The second one in the list). The -all package contains the source and the various lists of words that have changed, I am sure people are downloading this who do not really want it. I'll have to make it clearer in the next release.
I hope you find it useful. I would like some feedback (Use the sourceforge forums).
I am currently working on a Debian package, and am thinking about other formats such as OO.o2, kword and Abiword.