Over recent years the MET has been found institutionally racist and corrupt.
One of its own has recently admitted kidnapping, raping and responsibility for the death of Sarah Everard.
Many high profile murders and disapparances on its doorstep remain unsolved eg disappearance of Suzie Lamplugh (1986), murder of Daniel Morgan (1987) and the murder of Jill Dando (1999).
The HO has granted millions of pounds of tax-payers money to the MET to investigate the disappearance of MM. Was it realistic to think they could succeed when the PJ hadn't?
Officially David Cameron responded to the McCann's pleas for help to find their daughter, and so did the Home Secretary and the Metropolitan Police.
Unofficially it was said that a certain editor 'persuaded' Cameron to respond by threatening to harass the Home Secretary if he didn't. I expect there were negotiations whereby everyone got what they wanted out of the deal.
In my opinion Operation Grange, as a meaningful investigation, ended in October 2015 when it's members were reduced to just four officers.