In theory, aluminium foil which is completely sealed around the key should prevent someone from being able to commit relay theft. However, you can easily test this yourself by locking the car with the key, wrapping the keyfob in foil and then trying to open the car door with the key (in the foil) in proximity to see if the car detects the keyfob. It's a clumsy solution though, better to buy a faraday bag/signal-blocking bag off of Amazon for £7 or £8: kit.honestjohn.co.uk/top-tens/top-10-signal-blocki.../