Ummm,
If someone has highly enriched uranium, then they will build a uranium device,
that device is big, and needs a massive missile ( comparatively speaking ) to hit the target;
or one could mount that on an airplane, which can easily be intercepted by Iran's neighbors.
The real deal is to have a miniaturized device, and you need Plutonium to do that.
That said, Plutonium is a byproduct of nuclear fission; and it is not easy to machine
plutonium.
Actually it is very difficult to machine plutonium.