Even if it is left to 16 is it really OK to not question the girl who is been taken to another country to get married to a man double her age whom she has never met.
We're running around in circles.
We don't get to set marriage legislation in other countries. How they contract marriages is their problem.
There is existing legislation to deal with people being coerced into marriages abroad, with Prohibited Steps Orders the outcome. The legislation is fairly effective in dealing with removal from the country. The changes to the immigration legislation probably reduce the benefit to the perpetrators, too. So as things stand, someone can be prevented from being taken overseas to be married, or for any other purpose, up until the age of 21 and the marriage can be made ineffective for immigration purposes (both because of the "21 and £20k" rule, and more forcefully via the "primary purpose" test).
The case I cited is of a non-legal "marriage" conducted in the UK by someone who is not an agent of the state, which has no legal force or standing, where the participant in question is the subject of a prohibited steps order, but is also over 16 and claims to be a willing party. The issue of whether a prohibited steps order applies, or can be enforced, is a matter for the courts. But it's an incredibly invasive measure to injunct people to not hold a private ceremony which looks vaguely like a wedding, and the threshold for preventing that necessarily has to be high. It's perfectly legal for a sixteen year old to sleep with a man. If she refuses consent it's rape. No legal marriage was being contracted. All the rest is, to a great extent, window dressing.
We routinely hold thirteen year olds Gillick/Axon/Fraser competent to consent to, or refuse, potentially life-altering medical treatment, and we don't worry too much about checking that they're making those decisions in some sort of idealised bubble of perfect information, free from undesirable influences, mostly because we can't.
So I'm not sure what it is we want to protect people against. Being taken overseas for forced marriage? Prohibited Steps Order. Being married against their will in the UK? Plenty of legislation. Being slowly convinced over a period of years that they really want to do this, even though an external observer says it's bad for them? Well, we can stand around being outraged, but do we really want the courts intervening?