Let's put this to bed. This is a manufactured moral panic. Marriages of underage children essentially aren't happening in the UK. Here's why.
It's perfectly possible that you can find an illiterate imam at a benighted, primitivist mosque frequented by old men, who thinks that marrying children is fine and dandy. It's what he did "back home" and as he barely speaks English and knows nothing about British laws, he probably thinks he could still do it. With the slight problem that no-one has asked him to do it. Why not?
Well, what's the market for forced marriage? Why do you think Karma Nirvana are focussed on foreign travel in the school holidays? Why do you think the FMU is located in the Foreign Office, not the Home Office? The answer, of course, is that the main market for forced marriage is people wanting to bring family and clan into the country. By marrying daughters (usually) off to a Pakistani citizen, they obtain (at some point) right of abode in the UK. That marriage has to be legal in the country in which it's contracted.
So a sketchy imam providing a sketchy religious marriage in a sketchy mosque in Rochdale is of no value: it doesn't provide any form of right of abode. So the only people who might end up being "married" in such ceremony would be some bizarre hypothetical case in which two families in the UK conspired to "marry" (but not actually marry) their children for some nefarious purpose. If there's some underground culture of British citizens marrying their British citizen children off in shady illegal ceremonies for the purposes of, well, I don't know, "legitimising" (in their own eyes, at least) underage sex, then let's have it out on the table and see some evidence. Because I call shenanigans on that: I don't believe it's happening.
The problem of forced marriage in the UK is mostly associated with immigration, and therefore requires a legitimate (for some value of legitimate) marriage certificate. A religious marriage in the UK would, on many levels, not help that purpose. Hence it isn't happening. There are some ignorant, ill-educated imams to whom the news hasn't yet penetrated, who think that were such an event to occur, they'd be up for it. But it isn't, so they are only guilty of being ignorant, which we knew already.
If it's claimed that underage marriages are actually being performed, let's see the evidence.