It is possible to remove your rafter ties"loft" completely, but you would basically need to rebuild the entire roof system to incorporate a structural ridge beam, and convert your space into a cathedral ceiling structure. But you would need a structural engineer out, and to basically remove half the roof. And install some pretty beefy components. And even then, I would still worry about the walls moving. It would probably be easier, cheaper and safer to remove the entire current roof system and replace it with a scissor truss system. This would give you all the headroom you need.
Another option to improve the current loft would be to sister the current rafter ties with either the same depth LVL or even better a couple steel beams.
Another option to improve the current loft would be to sister the current rafter ties with either the same depth LVL or even better a couple steel beams.