Aside from little incidents like burning the White House in the War of 1812, the British have been wonderful allies for generations. Right now, I can think of three things the British can bring to the battle that will be tremendously useful. First, they have very good intelligence services with their own set of assets around the world. It always pays to have good information. Second, their foreign service can help us keep up the diplomatic pressure on the nations that harbor terrorists and conduct terrorism as a matter of national policy. Third, their military is highly professional and experienced at unconventional warfare. Specifically, I'm thinking of their SAS (Special Air Service), some of the most studly soldiers to ever walk the Earth. If you haven't done so, get a copy of "Bravo Two Zero", a first-hand account of SAS operations in the Gulf War. I'd also love to have some Gurkhas on our side. Gurkhas are legendary soldiers who are both respected and feared worldwide. When a Gurkha force deployed in the Falklands War, the Argentine Army promptly folded rather than face them, much to the Gurkhas' disappointment. The Gurkhas come for the high country and are very used to working in terrain like that of Afghanistan.

(a little info on Gurkhas)
http://members.aol.com/himimp/gallery.htm
http://newsvote.bbc.co.uk/hi/english/uk/newsid_364000/364960.stm