Usable head height is the single biggest factor in whether an attic can become a habitable room. As a practical rule of thumb, you want roughly 2.4m measured from the existing ceiling joists up to the ridge to have good options for a comfortable room.
With around that height, a Velux conversion keeps the existing roof shape and can still give enough standing room across the central part of the floor. The pitch of the roof matters here too, because a steeper pitch pushes usable standing space closer to the centre, while a shallow pitch spreads lower headroom further across the floor.
If you have less than roughly 2.4m, a Velux conversion may not give enough standing room to feel like a proper room. That does not rule a conversion out. A dormer raises part of the roof to create flat-ceiling height and floor area where the existing roof lacks it, and a hip-to-gable conversion extends the roof out to a gable end to open up volume. A mansard reshapes the roof more substantially again.
Bear in mind that head height alone does not make a room a legal bedroom; Building Regulations also require proper fixed stairs, fire safety, ventilation, and natural light. The only reliable way to know your figure is to measure it. We can do that during a free assessment, or you can begin with our suitability checker.