The real reason Britain is pulling out of Iraq is that we're getting the upper hand there, especially in the provinces where the Brits had a presence. And, now that they won't be busy with terrorists in Iraq, they'll have a freer hand to deal with terrorists in Iran.
UPDATE: Also, they can send some much needed troops to deal with Afghanistan. The fact that countries can get credit for sending troops, and then not allow them to be used at all (Italy) is slightly disheartening, though to be expected when dealing in a post-cold war NATO environment. The Brits and Austrailians are doing an excellent job in Afghanistan, as are we and the rest of the allies. Of course, there is much work to be done.
I still think that the primary good thing about the Brits pulling out of Iraq (aside from the fact that it implies that things aren't horrible there) is that now "we" (the allies) have a few extra troops that can be credibly used to threaten Iran. I'm not saying we should use them without a lot more provocation, but I think that a credible threat could be useful. If we can bully Iran into cooperating (carrots don't seem to be working, cause they apparently don't like the taste), then our job in Iraq and Afghanistan gets a heck of a lot easier.