Always glad to help.
Neither prime ministers nor the public can "order" investigation into free citizens. Any evidence found by a “backdoor” route against the McCanns would have to be excluded from a trial, as their lawyers know perfectly well. Therefore a trial of the McCanns using any such evidence would collapse immediately, since it is a matter of fact that there is no prosecutable evidence in the public domain as of now. Is that what you want?
But in any case much of this is just words. Making the McCanns "suspects", were it even possible, would have no impact on the case itself. So, apart from getting has-beens like Colin Sutton all excited, what difference does it make anyway? They cannot be forced to co-operate, they cannot be forced to answer questions, they cannot be forced to do anything except spend a few hours in the nick. It would, of course, make a very great difference to the way people, the public, felt, wouldn't it? Hidifacts and the rest would be out on the streets cheering and twiddling a fucking noose. Do you think that would help get a verdict?