I know that when I started building I was looking for insulated door blanks but it all proved too difficult so I went for expensive pre-made doors. And I know there have been various threads on here about making insulated doors. So I thought it worth while pointing out that I've just seen an ad in Passive House Plus for PHI-certified door blanks. I hope it's alright to post a link http://www.moralt-ag.co.uk/ferro-passiv.html They're available in the UK from James Latham. I have no connection with either firm, just thought it was something worth making people aware of. Neither have I any idea of the prices.

The allowable trimming does seem a bit awry, and whilst they make a lot of the sales pitch stuff, if you dissect the construction, its an engineered timber frame with a solid insulation foam core and plywood exterior, and a bit of metal. All of that is within the capabilities of any laminating company with a press.
The bottom rail too does seem light @ 70mm, -- or maybe I've read the wrong thing?
I guess it's the certification that gives them the edge, but pure construction wise it' doesn't seem anything special.