French Resistance in WW2
In WW2 the Germans forced the French to carry ID cards. Their use then didn't stop the activities of the French Resistance who continued to blow-up infrastructure and kill in the name of their cause. Some got caught but their 'terrorism' continued. So ID cards didn't then and won't today prevent terrorism.
The UK government wants us to believe ID cards will protect us by controlling immigration. People moving within the EU are not now classed as immigrants. Those not from the EU are a relatively small number. So ID cards do not help control immigration since immigration is now allowed but has a new name.
The government would have us believe ID cards will ensure state benefits are only received by those entitled to them. For years the National Insurance number achieved this function so ID cards again are not required.
But perhaps the real reason is that the state now wants to move to the next level simply because it can. Having sanctioned and encouraged ubiquitous communicating and information sharing it wants to harvest the benefits. Computers can monitor phone calls, email and internet traffic/usage without the use of humans, thereby technically not breaking any laws that require warrants to be signed. Computers already plot relationships between people, their thoughts and purchases. The UK government wants a new Generation 2 surveillance system, basically an upgraded version of ECHELON that the EU parliament has already investigated and questioned twice, at a cost that vastly exceeds any damage that could be inflicted by the terrorists and criminals it is intended to catch. But all this will be a waste of money if the government is not 100% sure who is at the end of the communication. That is why it needs ID cards - to close a gap in the chain.