Military loses control over London quarantine area after several months

Background situation

I’m creating a plot where the military set up a secure zone around London because of a crisis. I’m looking to explore what occurs when their authority begins to wane after some time.

What I’m trying to figure out

How could the military actually lose grip over such a significant quarantine area? I’m considering realistic situations like:

  • Issues with supply lines
  • Low morale among soldiers
  • Failures of equipment
  • Resistance from civilians
  • Breakdowns in internal command

Current approach

I have the general idea that they can no longer keep the perimeter secure, but I want to make it more convincing. What are the most credible reasons for this type of military failure? I need assistance in brainstorming specific events and a timeline that would result in them losing control of the quarantined zone.

Any suggestions on what the probable causes would be for this scenario?

Food riots inside the zone could overwhelm checkpoint guards pretty fast. Hungry people get desperate and military can’t shoot everyone.

Equipment failure would be a major issue. Military gear breaks down a lot and needs constant upkeep. After months of nonstop operations, vehicles would start failing and radios would stop working. They’d have a tough time getting replacement parts too. The cold winters in London would just add to the problems. Once communication falters between checkpoints, the quarantine perimeter would fall apart quickly.

Sounds like a cool setting for a game or story. I’d go with soldiers deserting to check on their own families after months of no contact. That would really mess up their numbers and organization.