The Discoverys were offered as "Kit boats" but I believe some were delivered fully built as well. (owner finished is not necessarily bad... depends on the craftmanship) The Coopers were never offered that way and would be factory finished.
I think there is a one-off somewhere around here of the 416 hull with a different deck/house plan that more closely ressembles the Discovery 42 - perhaps that's where some of the confusion arises.
I doubt you'll find a more voluminous boat than the Cooper 416 in its size/type range and it has much to recommend it in our soggy climate.
Another related boat, if you're not stuck on 40ish feet, is the North Sea 34, a very similar looking boat to the Discovery in a smaller package.. Sailnetter jrd22 used to own one before moving up to his Brewer 42, and I believe he has good things to say about it.
Nauticats are pretty nice, but I don't think they are the roomiest around, and come at a price (but with the quality to go with it)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)