I can't see the magnets being of sufficient strength to cause a problem. Even the knives aren't a problem in checked luggage although I would tell the check in agent that knives will appear during the triple-stage screening process.

If I was the check in agent, I would ask security to screen your bag separately with you present so that the airline is 100% certain about what is being carried and why and then let you go on your merry way. (In Heathrow's Terminal 5, G4S have a station in Zone E where your bag can be screened, or you can open it up and show them the items, discreetly. We normally use this screening station for firearms or if a passenger declares they are carrying something for someone else that they are unsure about).

If the check in agent doesn't do this, there is the possibility you may have to open the bag halfway between the gate and the aeroplane in the presence of police and security to do the same thing, but with more embarrassment, all because the x-ray scanners have sounded an alert without any pre-advice.