GPS was designed as an "unjamable" aid, it uses spread spectrum modulation and correlation techniques which make it very difficult to jam over a wide area. It requires some very clever techniques to effectively jam signals that are inherently below the ambient noise level; MOD want to find out just how susceptable it is and conduct periodic trials. Thats not to say they will be effective but you have been warned!
Interestingly it is illegal to "interfere with other wireless telegraphy" but as GPS does not operate in a frequency band allocated to radio navigation, it is considered as fair game.