All living spaces must have a floor to ceiling height of 8'. (With certain exceptions for sides of attics, and downstanding beams etc.)
Certain "non-habitable" spaces (i.e. ones you don't stay in for extended periods of time) may have ceilings at 7' - kitchens, toilets, bathrooms, halls.
Most attics are not designed to be converted into living spaces (despite the fact that it would have been easy to do when the house was first constructed), and so it cannot be marketed as a bedroom.
Nevertheless, a lot of estate agents call such convertions them bedrooms anyway.
There is also the problem that 3 storey buildings must comply with certain more stringent fire regulations - which would be the case if an attic to a 2 storey building were considered habitable.