and another thing….
the 'moulded' 4 (or 6) panel jobbies from wickes & the like have 38mm (at most) timber stiles (vertical edges) & rails (top & bottom edges), the rest is hardboard skin, filler (often hexagonal paper core – think of a wasp's nest) & a lockblock – and a further spine down the middle if you're lucky.
This is not normally a problem, but do buy the closest size you can get to what you actually need. If you have to shave more than about 5mm off each edge, get the next size down if you can
If you can, get fire resistant doors; the added benefit is that they're denser so less sound passes through. The downside is they're heavier.
Have a look at our range here: http://www.premdor.co.uk/
Wickes, Jewson, Travis Perkinst etc stock our doors so you shouldn't have a problem sourcing them. FD60 is our best fire rating, but even "Safe 'n' Sound" have some fire protection rating
If you were in a new build I'd recommend our "Speedset Plus" range, where the door is already fitted to the frame; remove the old frame, add the new one (with door), then add the architraves – job done. Not so good for older houses where right angles are just a fancy newfangled idea