sounds to me it was likely the UK customs, x-rayed and checked it and removed the knife because importing knives into the UK over 6 inches (at a guess) is illegal without the documentation. But then the customs would have put an inspection and removal notice on the package.
If it was China customs I also think they would put a notice on the box.
Perhaps it was just some lacky who needed a sharper knife, or it was a soap character out of ‘lock stock’.
I have sent plenty of stuff back to England, mostly choosing the overland rather than oversea option (just because I like trains) and the packages have never been interfered with despite going through Russia and eastern-bloc countries!