Time was when MI5 (or whatever they are called) wanted to listen in on your conversation they sent someone round to the local telephone exchange with some wires and a couple of bulldog clips.
The breadth of things that could be monitored was actually fairly large. I remember once, many years ago, being shown satellite photographs of the lake at the Chernobyl nuclear plant in the days running up the announcement that there was a problem with the reactor. The thermal imagery of the lake showed it warming up substantially over a few days. The information showing that there was a problem was being collected by our “security forces”.
The fact is whilst the data was there nobody saw it as it was buried in so much other information, photos etc, that you had to specifically been looking for it to see that something was wrong. The amount of personal information that they could gather about you was fairly limited and the number of people they could do this to was not huge. It was not a scalable system.
Nowadays the game has changed.