I have a family member who was in this in the 1980s in the UK. It was called Home Service Force (HSF). He was in the same force in Rhodesia first, then came to the UK after guarding against the insurgents in Rhodesia.
In the UK they guarded Key Point Infrastructure (like for example a bridge or a radio mast on top of a hill or a reservoir etc.) They would take a GPMG machine gun and dig in a trench, hidden in the woods. Mostly rehearsing for if Russian paratroopers or special forces to drop down.
I think the difference between the HSF and the TA, was the HSF would always rehearse going to guard the same place (near their own homes) so they knew the area very well. They did not even need maps. The TA would go to different parts of the UK to visit the regular army etc. The HSF did not involve the regular army, they knew their own patch and their own job without being told what to do. Plus they knew the local farmers. My family member knew someone in the UDR in Northern Ireland and he said HSF was a bit like UDR, but obviously just rehearsing, not actually fighting any real insurgents.
The HSF Coy in my battalion were on the whole a great bunch of blokes. When they got binned a lot of them stayed on as ......er "regular" T.A including one ex-inf cpl who became my Plt Sgt, and their CSM who was an ex-Cav SSM and a sort of posher, non-Welsh Windsor Davies, tache included and ran Training Wing or whatever it was called. Both top blokes.