" never really understood what the police were fighting for!"

I wasn't "fighting" for anything(It wasn't my job to fight), and never removed my "identification insignia"(I would have been disciplined), nor "went into battle".

If there were any under cover operations going on around me, I certainly wasn't aware of them but I certainly know that the regular CID, plain clothes officers weren't involved.

That's not to claim that such things didn't go on elsewhere though and I too have seen disturbing footage over the years.
However, in certain circumstances, even some more aggressive police tactics may be regarded as necessary and lawful but I'm not naive enough to believe that no individual officers ever over stepped the mark. Of course, if there is evidence of wrong doing, then they should be dealt with but it doesn't mean that such activity was common place or officially sanctioned.

I'm far from being "right wing" by the way, as many of my former colleagues would surely attest.