A kitchen rooflight is a working part of the house, so the specification matters. Steam, grease and sunlight all act on it, and the choices you make decide how easy it is to live with.
On heat loss, Part L gives roof windows the window limit and gives rooflights a separate, looser figure. Your quote names the category that applies and gives the rating.
Think about cleaning, too, and about what you can see from the worktop. A roof window frames the sky above the roofline, with the street out of sight. Keep that in mind when you choose between a fixed pane and one that opens. A roof window above a worktop gets splashed and steamed, and a pivot opening lets you reach the outer pane from inside. A fixed unit out of reach is harder to wash, and we discuss that when we talk about opening style.
Finally, think about heat. A kitchen already runs warm, and a south slope adds to it. A solar control coating or a blind is an easy addition at the order stage, and it makes the room more comfortable on bright days.
Glass finish is a further choice. A self-cleaning coating on the outer pane helps rain wash off dust and pollen, and a tinted or reflective layer cuts glare for rooms that face the light. Neither changes how the room feels at night, when the pane simply shows the dark sky.