In the case of Linux Mint, I can check the commit history, build the software myself and even validate it against public checksums. It is expressly defended against these types of attacks, making it an odd choice to single out.
So was I, because I have no idea what you're talking about so I'm curious about any more details to be able to look up why Linux Mint would be illegal in the UK. There's a myriad of laws it could fall under so undirected keyword searches won't let me find it and I'm also not sure if anyone can even read all laws that exist to see if there's anything related to what Linux Mint is/does, the question seems unanswerable but hints towards a certain thing being potentially illegal without saying what it is