I assume you can't or don't want to insulate between the ceiling joists which would be much cheaper? If insulating between the rafters I would go for a rigid board like Kingspan rather than the sprayed option. To get a decent depth of insulation and maintain an air gap under the sarking felt though, you may have to fit battens depending on the depth of your existing rafters. There's also a bubble wrap type product that can reduce the overall depth required but it needs to be used together with a board insulation to get somewhere near current regs.
Insulating at rafter level means you are heating the loft to some extent whereas at ceiling level you are only heating habitable rooms.