Hi all. I’m due to remove & replace some 4 panel doors in an old Victorian house. The frames are not of a ‘standard’ height. They are a lot shorter than 1981mm. Heights are 1930mm, 1940mm and 1970mm. 
Usually, in commercial buildings, I’d get a door blank and lip it after cutting it to size. 
I’m thinking not, but, am I allowed to trim the bottom of a fire door and fit some hardwood lipping to the bottom? Or does it need to be machined and made to fit the frame? 

Door blank is the way to go.  These are purpose-made for cutting to size and lipping but you must follow the blank manufacturers instructions as regards where abouts you can make the cut, lipping type, lipping thickness and adhesive type. 

A standard fire door will usually (with few exceptions) be unsuitable for trimming to these heights.