Filling an unfilled cavity is a great idea, but modern homes are required to have a cavity which is insulated with superior insulation panels that have an aluminium coating which additionally minimises heat radiation. This is impossible to install afterwards, unfortunately.
The recommended depth of loft insulation is 270mm, but achieving this is difficult in older homes with rafters only 100mm deep, particularly if the loft is boarded for storage. Foam insulation "boards" can be bought at DIY shops to be fitted beneath loft boards.
Floors are best insulated with carpet and underlay...
Old double glazed windows are not as effective as modern versions, which actually have their own energy performance ratings. The latest types of double glazed units have argon gas between the panes of glass (to reduce conduction and convection) and the frames themselves are insulated. The glass is also coated with thin films to better 'trap' infra red radiation from inside rooms, yet still allow heat radiation from the Sun to get inside.
Heating systems on appropriately set timers with thermostatic controls are efficient and even more so with TRVs. But to gain a Good or Very Good rating, a boiler Energy Management system is needed, which can be linked to the seasons as well as the outdoor temperature.