I think in fact it's relevant to the specific weapons system you're talking about and the number of things you get. Obviously, with the smaller number of things you get—in this case, up to 12 submarines—cutting it in half makes little sense. If you're buying a large number of things, dozens and dozens of things, then it may or may not make sense, depending on what you want the thing to do and the threat. There are a whole bunch of other considerations.
It may or may not make sense. I tend to agree with the Prime Minister on those points.