Spot checks on tube and train stations are becoming quite regular these days ... we have them in Cardiff every time there is a match on. I would suggest if you are carrying a blade then declare it straight away .... don't wait for them to find it as they may think you are trying to conceal it.- what's situation with carrying it in luggage on tube to get home?
If it's securely placed in the bottom of your baggage, where you or someone else cannot get to it quickly or easily, you should be fine as long as you are going straight home.
Ed