Hooray, another Brit! I've seen black batting in some of the online shops, but we just don't have the quilt shops that they take for granted in the US, do we.
How about a not-terribly-bright colour such as blue? There are greyish blues around, that would be nice and subtle but still just enough for the thread to be visible against all the fabrics. I think grey could work but would be a bit dull. Have a look through the threads at the Cotton Patch, for instance.
http://www.cottonpatch.co.uk/acatalo...g_Threads.html