There’s a term known as an 80% lower. The lower receiver of an AR style rifle is what’s considered the firearm. By selling an 80% finished product, the seller skirts pretty much all gun regulations as the person isn’t selling a gun, and the buyer has to complete the last 20% of manufacturing which transfers the ownus of the legality onto the buyer/end user. In the USA it is perfectly legal for an individual to manufacturer a gun for their own personal use so long as they are not manufacturing for later sale.
Each country has their own importation rules and regulations. Most likely UK customs will have their own definition of what a firearm is, and will likely not allow an import of finished or partially finished gun parts.
The FGC9 is a 3D printed gun, developed by a German, specifically for people who do not have access to firearm parts. The FGC9 relies entirely of off the shelf hardware store parts and has been tested to fire multiple hundreds and in some cases thousands of rounds accurately before failure.