These suggestions come from a consultation with a client who had a detached property with a dual pitched roof, and neighbours who have taller houses.
You could literally increase the height of your roof by building up a few more brick courses above the head of some windows, and then put the same pitch back on top. This is possible if the house is detached and the neighbours have taller houses- so there would be president in the street for a taller roof line.
If the property is Victorian, with floor to ceiling heights of 2.7m, you could have scope to lower your ceiling to provide attic space.
To meet building regulations, you will need 2m height at the top of the stairs. To achieve this, you could put a flat roof dormer along the back, providing more usable space, but you will still have to raise the roof slightly.
Alternatively, you could change the pitch of the roof: make the roof steeper and flatten the top to make a crown roof/ Mansard roof. This roof might still have to be taller than what you have at the moment, because you will have to thicken the floor joists, costing 15cm, and you will have to add insulation, costing 5 or 10cm, so you will have 0.2m less usable space.