For gods sake we already have more troops in Afghanistan than the next two contributors combined (albeit a tiny fraction compared to the US forces, but then it's their bloody war.)
We shouldn't be spending billions of pounds buying new equipment for more troops, we should be drawing down troop numbers to a level that we can afford. If that means other nations taking up some of the slack, then so be it!
We need our armed forces to be geared towards the UK's interests, not to be tailored to what the Americans will find the most useful.