They have to--he's the key to their "Bush Lied, People Died" mantra. Wilson is the dupe looking for the ball behind home plate in my little analogy. The British government is the fan that found the ball (the Brits stand by their story that Saddam bought uranium from Niger).
It even goes deeper than that. It's beginning to look like Wilson was just the point man in an effort to discredit Bush in his effort to go to war with Iraq. If Wilson goes down then a lot of rats follow him.
Probably why Kerry is try to distance himself from Wilson removing all ties to him from his website.