If John McDonnell wants to disband MI5 and disarm the Police, then who does he think that he is? Peter Hitchens?
MI5 told Tony Benn to his face that it would murder him if he ever became Prime Minister, an office to which he would have had to have been appointed by the Queen.
It devotes much of its time to harassing and intimidating trade unionists, as do the existing armed sections of the Police.
Those are not necessarily arguments for disbandment. But they are concerns. And those concerns are now shared and expressed at the very highest levels of the Official Opposition.