Rob: For me, as a liberal, it's about balancing what's likely to curtail freedom most: in other words, I think it's about weighing up the likelihood of someone taking someone else's freedom (to survive, for example) away with a gun versus the idea that people should be allowed to carry a gun. I think the UK's fairly strict gun control is right; we don't totally ban guns, and indeed I wouldn't support a total ban on all guns ever, but we do require people to have strict background checks. And that's pretty successful at ensuring we avoid gun massacres, which are a much larger impediment to someone's freedom than "you need to fill in these forms before we let you have a gun". I don't feel that is all of a muchness with the risk of identity theft, just since when someone's dead it's rather more immediately permanent.
I guess I should note that this way of looking at things isn't universal among liberals: part of what it comes down to is a split between liberals who are only liberals versus the state (perhaps these could be described as libertarians), and people like me who are liberals versus other power sources as well. So libertarians would tend to be more in favour of, for example, lower taxes, right to carry, etc, because higher taxes and removing guns is the state encroaching on an individual. Social liberals like me would tend to be concerned too, though, with the fact that individuals can oppress and remove the freedoms of one another - so will often advocate things which encroach on the liberty of one individual in order to protect and safeguard the liberty of another. Which isn't ideal, but in my view private power is as dangerous to freedom as public and state power is most of the time, so there needs to be some way to counterbalance that.
CG: Assault rifle was probably the wrong term, but these massacres are carried out usually with things that are a lot more dangerous than most of the guns available in the UK. And sure, the person may be the root of the problem, but a nutjob who only has a knife in a kitchen drawer will statistically be likely to kill far fewer people than a nutjob with any sort of weapon that's semi-automatic or has a sizeable magazine. As I say, I have no problem with you guys deciding you want guns anyway, but it's simply not true to pretend that all the mass shootings you guys have would happen anyway with or without easily available guns. We do have nutjobs over here too, but we don't have regular gun massacres.