I can never remember being told there were no political police in Britain? A Citation for when you were told that would be cool.
Notwithstanding that all police are political and are funded with the underlying principle that their primary purpose is the defence of the state. Right the way back to Sir Francis Walsingham and Thomas Cromwell the modern state had had agencies than nor only have the powers to surveil opponents of the state but also to take executive action, arrest, charge and at times punishment up to and including death. When was this ever a secret? The power of such organisations relies on people knowing about and fearing them. The MPS only had 40 years between establishment and the establishment of the Special Irish Branch and I have little doubt there were little teams reporting to the commissioners long before that.
The Security Service may have started its modern guise as MI5 in the 40s but no serious historian would argue HMG hasn't had that capability, however amateurishly delivered, since there was an HMG. And of course how else did we manage to control an empire that covered a quarter of the world without really effective political policing?
Maybe the Ladybird book of the British Constitution might not cover this but I can't think of any rational sources that would have claimed at any point in our lifetimes that the UK had no political police.