This kind of situation can only be sorted out if clear and bold rules are introduced.
If it is the case that people under 21 years of age who marry are likely to be participants in a ‘forced’ marriage or a marriage of convenience, then the age for recognised marriage in the UK could be increased to 21 for everybody, citizens and non-citizens. There is nothing to stop young people living together and only tying the legal knot at 21.
If a British citizen marries a non-citizen there is rightly an assumption that the couple can live in the UK. Of course this is something that can be abused and there is not much the state can do about it. Yet it is possible to stop the offending British citizen playing the same ruse twice.