As Boris Johnson was the MP for Henley from 2001-2008, Mayor of London from 2008-2016, and has been MP for Uxbridge & South Ruislip since 2015, I will assume that you have confused the present Prime Minister with a less experienced MP, Noodlenosefraggle.
Given his political chops, Boris certainly can't be confused with Keir Starmer who, prior to becoming MP for Holborn & St Pancras in 2015, headed the Crown Prosecution Service (CPS) from 2008-2013, during which time that organisation found insufficient evidence to prosecute Saville on more than one occasion.
As a matter of public record, after Sir Jimmy Saville died, Sir Keir Starmer told the BBC "I think this is a very real issue that has been with us for a very long time. You can go back to the 50s and probably earlier to find examples, but you can find many more recent examples, and it's a very simple proposition: if you're in a position of authority and you have cause to believe that a child has been abused, you really ought to do something about it".
As my earlier post has been deleted for no reason that I can fathom, I must assume that the spirit of Momentum lives on in what will no doubt be presented to the electorate as a shiny new Labour Party.