This is just a guess based on my experiences, but I think that Hardcastles are Dunhill seconds. Meaning the line is set up to run Dunhills, but somewhere during production they notice a spot or a pit or something that doesn’t meet QC standards so it’s set aside and refinished later (probably to lesser standards) as a Hardcastle.
Parkers are a sub-brand of Dunhill, meaning that the line is set up to run Parkers. The wood used is probably of a lesser standard as are fit and finish, but they were intended from the very beginning to be Parkers.
I write this because as Hardcastles are initially intended to be Dunhills, use the same wood and at least some of the same fit and finish (until the flaw is revealed), the price point is higher than Parkers, which are “lower shelf” from the very beginning. Like I said, this is just a guess based on owning several of each over the years. If anyone knows better, please correct me if I’m wrong.