Thanks Guys.
There doesn't seem to be much information on this... however I have seen one site which says you can't due to humidity levels???
External doors are built with 18% humidity (Internal doors 10%). Thus an external door will warp when it drys out but an an internal door fitted...
After searching for over a year for internal doors that both my partner and I like we finally found some but they are external oak doors. Can you hang an external door internally?