Thanks to @mia_out on Twitter, I've just found out about:
http://osconsult.ernestmarples.com/
"The Government have launched a consultation on how to make Ordnance Survey data free with no restrictions on reuse."
Full details are given in: http://www.communities.gov.uk/documents/corporate/pdf/1411177.pdf
The deadline for responding is 17 March 2010.
What do you think would be the best approach to take here? A single official comment from Wikimedia UK? Persuading lots of Wikimedians to emphatically say "yes"? Both of these? It isn't clear to me whether they want detailed, logical responses or large numbers of responses saying the same thing, although I haven't read through the whole document yet...
Mike