No, there is no minimum height requirement for a loft conversion stipulated by the building regulations. It is often cited by experts that the minimum recommended height is 2.2 meters though this is by no means a hard and fast rule. Certainly 2.4 meters is a very good height for a loft conversion.
To measure the available headroom measure from the existing joists to the apex of the roof but remember that the new floor will be at least 175mm above the existing heights to factor that into your calculations.
It should be mentioned that the measurements noted above are assuming that a dormer will be added. If you only intend to add flush fitting skylights than, as a guide, 2.8 meters at the apex would be a suggested minimum.
