Hi all,
Personal opinion: The best place to discuss UK-specific text, including for the proposed petition, would be on-wiki at:
http://en.wikipedia.org/wiki/Wikipedia:SOPA_initiative/Proposed_Messages
since it would probably have to be a community-driven action rather than a WMUK/WMF one, as with the other non-technical aspects of the blackout (but feel free to disagree with that. ;-) ).
Thanks,
Mike