To the best of my knowledge there is no minimum head height for any of the circumstances you have listed. The only place headroom is governed is over stairs where you must acheive min 2metres above the pitch-line of the stair nosings. Therefore if the headroom in your loft conversion affects the top of the stairs then you need 2m here.
Other than that its just personal preference to what height you need! In new-build situations we normally allow 2.4m (8ft) ceiling heights.
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