My advice to you (not meant to be ironical/cynical) take your loss and go on.
Even in US there have been lawsuits about spring assisted openers, and the outcomes were never identical and hardly ever predictable. Problem is: prosecuters and judges are lawyers, and those are great with words, but not always with practical things. Sometimes they use words/terms that are not applicable to the item they are discussing.
UK lawmakers are even more “concerned” about the safety of the public. Even eating an apple in a public place using a knife that is not very obvious a table knife can get you into trouble. Summary: the institution that has your knife does not want you to have it.
The (financial) pain lies in the fact that you are allowed to buy a knife from a country where is it legal to sell such a knife, but you are not allowed to use (or even own) such a knife in your own country.