As many as their prime minister is willing to surrender. Americans aren't forcing the British to help them, apparently their govt thinks our wars are important enough for them to bring forth their own soldiers.
In other words, if we're stupid, they're stupider for following us.
EDIT
It's depressing really, that in situations like these all the ppl of your country can do is complain.Your soldiers are out there trying to make the world a better place. What are YOU doing? Recycling?
You insult the US, but when it comes down to it we're the ones with the big toys and we use them to protect you. Terrorism affects everyone, not just the US, your military sees that, and that's why they are out there helping us. It's ppl like you that make them look bad, sitting in your drawing room drinking tea and on the computer to kill boredom.
Theory is nothing if you do not put it to practice. If you remember it's this kind of attitude that has gotten, much of Europe in trouble in the past, when something happens turn you head the other way and pretend there's nothing there, you decide to take action until the enemy is already on your doorstep, which it has, I do recall a terrorist bombing in London 2005.
This war is necessary to help the afghans better their lives and catch the granddaddy of all terrorists. You can be an idealist if you want, but you must know that problems aren't solved by complaining and pointing fingers at others. What? You think by wishing the bad ppl away everything will fine and dandy? No, it's men of courage that make it happen, your own men, men that you are insulting by calling their efforts pointless.