Hello all - I have had some positive news. I left a comment using the Apple Feedback Page, and it appears that sparked something where someone within Apple took another look at my case.
After doing so they found that the delivery driver had in fact now been let go from the Courier and it appears they finally understood I had my original iPhone stolen and replaced with some Android Fake instead.
This turned around completely from them being unhelpful, to being very apologetic and very helpful, and I got delivered a replacement iPhone 15 Pro Max (this time i opened the box in front of the delivery driver and checked the Serial Number against the Apple coverage checker website). I wasn't taking any chances this time.
I suggest those still going through this should use the feedback form on Apple.com/uk to report your problems, it can't harm to express your views on there.