The UK doesn't need to put restrictions in for 3d printing guns because the viable approaches for 3d printing them usually require _some_ off the shelf gun parts not to mention actual ammunition which you can't feasibly acquire in the UK to begin with.

You can acquire guns, gun parts, and ammunition quite easily in the UK, and entirely legally.

You need to hold a suitable licence, which isn't expensive and is mostly an exercise in proving to the police that you're not a violent psychopath who's likely to run up to people in cars and shoot them in the face.