Well there may not be a specific visa for it, but it definitely happens. I've read of lots of instances where minors have come into the UK under some sort of irregular arrangement to live with people who are not their parents. Two that spring to mind are the very high profile stories of both Victoria Climbie and Mo Farrah. Mo Farrah was brought into the UK at 9 years old, by a woman he'd never met, using a passport that wasn't his and he proceeded to live with her for the rest of his childhood. Victoria Climbie was brought into France and then the UK by her great aunt, who claimed to be her guardian/adopted mother but there was no legitimate documentation to support this.
Look, I am not particularly interested in the finer detail of how the 14 year old Pakistani bride arrived in the UK and on what sort of visa it was. It's irrelevant. The point of the post was not how she arrived, but the fact that she was married (Islamically at least) at only 14 to someone she barely knew, and that poster's colleagues refused to be judgemental about it on the grounds of her culture.
I know very well that in very conservative Muslim communities there is often a great rush to marry young people off as soon as possible, because it's one way of making sure they don't start having sex outside of marriage. I know a Jordanian woman whose son got married at 18, to a 16 year old. (an Islamic marriage, not a legally binding British one.) She told me 'we like to see them get married young in our culture. It's easier that way.'
Whatever the truth of the matter, it's absolutely not beyond believability that this girl did marry at only 14 and did end up in the UK with her 18 year old husband. Anyone who knows the truth about conservative Muslim communities would be a bare faced liar if they tried to insist that this never happens and could never happen.