First I wonder how exactly they found out about these "gun parts" this guy had to start the raid to begin with. If they can be so secretly made in the privacy of one's home then I'm assuming they were there for something else and just happened upon them. It really bugs me when news stories say "Police found X in mans home during raid" X being something which could be kept inconspicuously without anyone knowing, and never say why the police were there to begin with.
So essentially they came into his house saw some pieces of plastic on the table and immediately assumed "gun parts" and charge him with possession. Despite the fact that those parts could really be from anything. The police themselves admitted they have no idea what the parts are for.
Lovely place the UK.