BBC 4 / iPlayer is pretty good for several that have been mentioned here. The Singing Detective is currently available. Our Friends in the North was there a few months back (OFitN was, in my recollection one of my all time favourites, and remains remarkable in its ambition, but I was surprised in my rewatching of it how dated it has become despite resonant themes of institutional corruption; compromise in the Labour Party and marginalisation).
It is also interesting to rewatch series that were groundbreaking at the time, but their influence now makes them appear somewhat formulaic - This Life for example.
I'd like to see the first series of Big Brother and Castaway 2000, just to see how far reality TV has morphed in the last couple of decades.