It's not the govt., it's the spooks that want this. Most politicians barely know (or care) what a sim card is.
I can think of 2 reasons why mobile phones are impt. to terrorists - communication and bomb fuses
Yes, this measure won't defeat the most determined terrorists, but it will make raise the barrier to entry for "muppets", like the one who turned up at a restaurant in wales last year and tried to blow himself up, or the pair who drove into Glasgow airport. Or Richard Reid. Remember - it might not stop them from carrying out an attack, but the registration of the sim will be valuable during the subsequent investigation helping the spooks prevent future attacks.
I used to think that these seemingly superficial measures were pointless, but having done some research into terrorist incidents over the past 30 years, a surprising number were carried out by what I would call "muppets" - i.e. careless amateurs, so I think simple steps like this may actually be of some use.
And lets face it, that fits with intuition - are you more likely to have a high or a low IQ if you decide to blow youself and innocent civilians up?