When the rooflights need to comply with Building Regulations, what should the minimum U-value be?

Building Regulations Approved Document L2B that covers existing buildings other than dwellings requires rooflights to have a minimum U-value of 1.8W/m²K when assessed in the horizontal plane. The guidance given in BR443 provides for a 0.3W/m²K adjustment to a triple skin rooflight giving a 2.1W/ m²K minimum value. Hambleside Danelaw quote all U-values in the horizontal plane unless otherwise required.