Most satellite signals are high frequency, but low power (it's amazing how little power you need for a LoS radio signal). So generally they're easy to overwhelm, especially as GPS receiving antennas tend to be multi-directional. GPS transmits on several fixed frequencies and is relatively bitrate, too. The military signals are apparently more robust, but full details are more scarce.

the article mentions that satellites are being jammed as they move above Britain, what does GPS have to do with it?

GPS jamming is for recievers.