Re-roofing with the same or a visually similar material is normally exempt maintenance. Changing to a markedly different material or colour, altering the roof shape or pitch, or working on a protected structure can require permission.
A slate roof replaced with concrete tiles of a very different colour and profile could be argued to materially change appearance, particularly on a terrace or in an area with a consistent roofscape.
A re-roof is also an opportunity to address insulation, ventilation and the underlay. Retrofitting insulation at rafter level requires attention to ventilation paths to avoid condensation in the roof void.