=v= The fine folks in London have tipped me off to their new
website for the Central London Critical Mass:

http://www.cm-london.cjb.net/

Their site mentions a number of active rides in the U.K.

=v= Until 1999, there was a Critical Mass U.K. site that listed
38 active rides and 1 dormant one, but it was wrecked by
hackers, and currently only has a years-old May Day invitation
on it. I went to the "Internet Archive Wayback Machine" and
found the pre-hacked site:

http://web.archive.org/web/19990503120005/http://www.critmass.org.uk/index.html

The full history is here (warning, it relies on Javascript for
navigation!):

http://web.archive.org/web/*/http://www.critmass.org.uk/

=v= I did a little foraging and found a few more websites that
are updated (or, at least, a bit newer than 1999), so now I've
got the restored list of U.K. cities at www.critical-mass.org:

http://www.critical-mass.org/europe.html

Some of these rides are dormant, others have been absorbed
by cycle campaigns, and others now only ride during special
occasions like May Day or Bike Week. Details are always
welcome, of course.
<_Jym_>