We'd suggest either between and above the rafters or between and below the rafters.

Most of the time the existing rafters are not deep enough to only add insulation between and achieve the levels of insulation required by Building Control.

If you are re-covering your roof, this would be the time to add insulation. Adding insulation above the rafters during this process is very simple and can also mean that you do not need to use a breathable membrane over the rafters (as wood fibre boards do the same job) which is particularly beneficial if you have bats.

However, this does mean that the roof covering will be lifted by the thickness of the boards which may not be an option on a semi-detached or terraced house, unless you have the brick party walls protruding through the roof line.

Using the flexible wood fibre insulation SteicoFlex between the rafters and an insulation board, such as Beltermo Ultra or UdiTOP, above the rafters gives the best performance overall. These boards change temperature very slowly and resist the high temperatures from the sun in the summer months, preventing this heat from entering your roof space, keeping it cool. They also retain heat and buffer sudden drops in temperature in the winter months. 

If you do not plan to re-tile or slate your roof, or cannot alter your roof line, then the only option would be to insulate between and below the rafters. You will still achieve the same U-values, same levels of acoustic insulation but you will have the option to lime plaster the surface of the wood fibre boards if you’d prefer.