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.
>>chippi+(OP)
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.