There's no such thing as a Royal Mail "naughty list" so you definitely don't need to worry about that. They don't have the legal authority to enforce drug laws and its not their responsibility. Absolute worst case they'll pass the package to the local plod but 99.99% chance its just stuck in a sorting centre.
We don't live in an American dystopia.... well not fully and not yet at least.
We don't live in an American dystopia.... well not fully and not yet at least.