Hi Stephen - You say you are replacing old council house doors, which I assume are the old imperial 2'6" x 6'6" size (72mm x 1981mm).
Because of the amount you need to trim, I'm worried that the new doors you've got are metric ones, which would in fact be 762mm x 2040mm. These are called metric because they're designed to fit in the frame openings of 800mm x 2000mm found in more recently built houses. Also they normally have a thickness of 40mm instead of the old imperial size of 1 3/8" (35mm).
If the doors you need to trim have real timber stiles and rails you should be OK trimming them, although you might invalidate any guarantee you had. An electric planer should be OK on this type of door.
However, If the new doors are the pressed moulded hardboard type, the actual timber stiles and rails only go in about an 1 1/2" or so, depending on the manafacturer, the rest of the internal space is generally a type of expanded honeycomb paper. You could seriously affect the stability of this type of door if you trim too much from them. They are only designed to be trimmed a few millimetres.
Another thing, as well as affecting its stability; if you trim too much off, you might be unlucky in planing it a lot as you may start to hit staples.
So before you start planing away, Id be interested to know the actual size of the new door you've got. Is it just the one door you need to trim, or is it all the doors? If this was the case it would certainly suggest you've got metric sizes instead of imperial.