It's fairly easy to make your own knives without said GPS tracker. And how many million knives are already in existence without the tracker. Maybe we need to have compulsory knife registration but most homes would have several kitchen knives so I suspect it would be quite a task to setup this database and keep it up to date. Every knife currently in ownership would need retrofitting with a GPS tracker (which is probably not going to be possible - I have a set of kitchen knives which are single pieces of steel) and every knife bought, transferred or disposed of would require an update to the database. I don't think it's very realistic really. Apart from anything else knife crime (whilst it has been increasing) is still lower than it was in 2006.