There's no earthly reason for the police to have taken the action they have taken. Get a solicitor, Citizens Advice will provide you with help if you can't pay for one, and don't go alone to the interview. From what you describe of this incident, the police have no grounds whatsoever to keep your knife and certainly no reason to destroy it. You are in a very much stronger position than many here seem to think - you have done nothing wrong! so fight them and demand your rights, not request them! If I were in your position I'd leave absolutely no stone unturned to get this sorted in your favour, and be sure to get any warning removed from the record.
The behaviour of the police in all this is not only wrong but inexcusable...........this sort of thing makes me fume with anger........they've succeeded in getting you on the back foot already and you needn't be - you are in the right, so make 'em swallow it!