Er, my guess is that the "King Post" and the other timbers joining it
make a truss which supports the roof - the rafters resting on wall
plate and ridge with one or two purlins in between. Where the
horizontal truss timbers overlap they are almost certainly firmly
bolted together.
So the whole roof+truss are independant of the ceiling joists which
may be removed however you want. The 2 ceiling joists which cross the
end of the proposed opening would need strengthening as they would
carry the extra load of the ceiling no longer supported by the removed
joists. Perhaps double up with a new timber on top bolted through in
several places.

cheers

Jacob