I would not opp for raising the ridge, as it will require planning permission, which in my experience it would not be approved due to the planning regulations. Especially if it is a terrace property and all ridge lines are at the same level. However, if it is a semidetached property and your neighbour wishes to do the same development, you may opt into a section 106, were both properties are constructed simultaneously. If you do not wish to remove the ceilings on first floor only option will be to.
Construct new steel floor, to run parallel to ceiling joists instead of the conventional perpendicular from Party Wall to Party Wall, and lower steel between Joists, 25mm above ceiling, this will help you to gain the acceptable height level in the loft.