The old Labour Party vilified him for his actions as a pre-WWI Home Secretary to keep the peace during a labor dispute. They ignored the fact that he always sympathized with efforts to make sure that all workers were treated fairly.
He was vilified by extreme reactionary Tories for his his Liberal Party alliance with the Radical firebrand David Lloyd George. This despite the fact that Churchill only wanted reform which preserved the English system, not revolutionary change.
The Conservative Party establishment in the 1930s vilified Churchill for his strong stand against Hitler and his realization that appeasement and hoping Hitler and Stalin would destroy each other was not good policy. There are still proponents of this criticism today in people like author John Charmley whose criticism of Churchill includes an undercurrent of anti-Americanism. Charmley gives the impression that the UK being a "junior partner" to America is as bad as being subservient to Hitler. And anti-Churchillism is associated with anti-Americanism by critics on both the English right and left
And in today's PC world Churchill is downgraded because he is too much of a reminder of the old heroic greatness of the British people. If people are taught too much about how only 67 years ago their ancestors stood up to the fearsome juggernaut, they may see an alternative to the limp-wristed cultural surrender so prized by todays PC elites.