Find, follow, attract, use front groups (peace, human rights, anti war) and attract interested people in and give them safe busy work 'issues' over decades.
Anyone with any state connection would have understood network tracking since about the 1970's know to avoid any of the new hardware of software methods.
The other issue the UK has is CCTV and cell phone tracking. Any member of the press, a lawyer going out can be tracked and any meeting they have can be noted.
Networked office equipment, new office computer equipment that arrives been ordered online is another way in.
It all comes back to staff numbers the UK can offer good wages to for the skills of tracking most people within the UK for decades.
Hire too many new staff with skills and the UK cannot trust the vetting. The UK has a long issue with trying to secure its own gov workers.
The UK faced staff issues, working conditions and wage claims from its gov staff over many decades (1950's-early 1980's).
Growing the staff count to watch all of the UK all the time might be a budget and vetting issue long term.
Why watch everyone when the UK has front groups that can draw people in?
The UK will have an interesting question soon, find the cash to pay contractors and gov staff to watch the entire nation or find a very easy way to watch front 'groups' in the community.
Front groups are very tricky to keep public enough to attract new members but well away from the press and citizen journalists asking questions. re "Or they can just go into the woods and whisper to each other." The UK would offer both people a lot of cash/deals. Cell structures are easy to turn given time and a lot of cash. Once a few cells get turned the entire can be uncovered and turned.