MReyn, true, but he can raid the contingency fund if persuaded. It's happened before.
Toddbabe - fair point, but if we're being constitutionally pedantic, a legitimate case can be made for the Leader of the Opposition to go out there- as, indeed, he did. Since Brown is more likely to be the next PM than Cameron (unless the Labour party does the unexpected and chooses someone else), then it seems a bit odd to exclude the Chancellor who's far more important at the moment governmentally speaking than Cameron.
Furthermore, which is better - the military leadership telling the Chancellor his money is being spent appropriately, but more would be nice, please, or the Chancellor going out and seeing with his own eyes that the MoD (contrary to perceptions in the Treasury) can spend money effectively, and deciding that more would be justified? While I never, ever thought I'd ever finding myself offering a defence for Gordon Brown in this case, I don't think that his visit can just be dismissed as nothing more an outrageous publicity stunt that's going to be of no value to those fighting out there.