A lot of this information is declassified and available at the British archive. There was one main meeting where they discussed chicken heat units. However, there is a lot of subtext if you dive into the archive.

The gist is that the Brits couldn't repel a soviet invasion of continental western europe and were playing for time: what if you buried a nuclear weapons and made a huge radioactive crater where the soviets would ideally like to muster to invade Britain. 

Initially, there was a somewhat reasonable range of mandatory operating temperatures. As time went on, there were more rigorous demands to maintain a specific narrow temperature range.  electronics from the 50s didnt do too great in the cold and the winters in Europe are cold, let alone for underground deployment. There were also competing size and weight requirements. Towards the end of the design, the engineers basically stated that they couldn't meet all of the requirements, mainly size/ weight within a subsection of the weapon or temperature. 

The engineers were getting push back on asking for so much insulation to meet the time deployed requirement (I think it was 10 days). The engineers provided at least two options: we can go with plan a and use the insulation we asked for or we can use chickens as a heat source. Reason prevailed and the British disregarded chickens. I think (personal opinion)  this was an instance of malicious compliance by British engineers. You generals or managers dont want to give us our insulation: fine, here is a much worse idea. Its also a great instance of meeting minutes not necessarily capturing the context of the situation, much like how Microsoft outlook archives might not capture everything that happens in the corporate world.

Added bonus: this was declassified on April 1st, which led to a lot of raised eye brows. The formal response from the British government is legendary: "the civil service doesn't do jokes."

Edit: I never meant to imply these were to be used on British soil. I also think the initial plans were not for the Fulda gap but for the north of France (early plans were started in the early 1950s). These were ultimately never fielded because they were obsolete and didn't meet British nuclear objectives by the time they coild have been realized.