Hi,
The only conceivable value in .cm is in whether
or not it gets hits intended for .com. So, if
your name does get hits that were meant for the
london.com domain then some would argue that this
is a 'value'.
However, as a promotable domain in it's own right,
london.cm is no better than e.g. london.tk or any
of the other ccTLD's.
As for value, it's very difficult to say, and there
are no well-known sites that use the ext'n .cm.
I'd like to say that this domain is worth much more,
but in my view, and from a liability stand-point, i'd
say it's worth reg fee...
Good luck,