I'm not sure PACE and the PACE Act itself are quite germane in the context of decisions about firearms deployment (as opposed to entry to dwellings and premises), which I suspect will be more a matter of policy.
It requires further research, but in regard to the decision of response officers not to enter the dwelling without armed support, you may wish to consider section 17(1)(e) of the PACE Act:
Quote17. Entry for purpose of arrest etc.
(1)Subject to the following provisions of this section, and without prejudice to any other enactment, a constable may enter and search any premises for the purpose—
...
(e)of saving life or limb or preventing serious damage to property.
Whatever else can be said, no-one can argue that a police entry would have been illegal (not that any legal repercussions would have flowed from it even if entry had been illegal, as a breach of PACE was surely condonable in those circumstances).
I think section 17(1)(e) does further somewhat undermine guilters' arguments about Jeremy's conduct that morning. You say that Jeremy was trying to delay them. Putting aside the illogicality of that argument, it seems to me that if Jeremy was having an influence on them at all, it was to persuade them of the potential danger of the situation. Surely you must acknowledge that Jeremy could not have known what they would do next? Firearms officers could have arrived and started raiding the farmhouse within minutes.
There was likely to be a policy. Whether it just took the form of a policy document or some sort of extensive operational manual, research is required to discover.
Currently, as at 2021, Essex Police Use of Firearms Policy published on their website. It's a laconic document. It may sit in front of a manual, though, and that could be a 'protected document'.