The Hagn is definatly not for the budget minded. My thoughts on the actions are well detailed on the website. The extraction works equally well with rimless cartriges (maybe better) and while I also prefer rimmed in single shots, the two I have commissions to build are in .280 and .375 H&H.
As for the .375Flanged, the only sence this cartridge makes is British nostalgia for older, weaker actions. I'm nostalgic minded, and if I were working with an older action it would be a consideration. To each his own. the Hagn and H&H cartridges seem very "right" together.

The problem with the Hein is the same with all the "new" single shot actions I've seen come and go. The firm will have to get several in the hands of custom gunmakers before they can be adiquately evaluated. Many of the past companies have folded before this happened. Only time will tell.
The prototype is for dinosauer cartridges which doesn't appeal to very many, as they will find out. Although they are talking about multiple sizes, personally, I don't want to experiment with thier experiment. Scaling up or down can be challenging.

As for stocking a Hagn, do find someone with experience, the actions deserve the best in stockmaking.