Physcia tribacia

When found on site it was suggested that this lichen was Physcia tribacia. I collected a sample and had a go at keying it out:
Getting to Physcia was achieved by the following characters:
Foliose lichen
Photobiont which is chlorococcoid (trebouxioid) and not cyanobacterial.
Lobes that are spreading rather than strap shaped and not hollow
Lobes lacking tomentum on the lower surface (felt like mat of fungal filament known as hyphae).
Whitish to pale grey coloured lobes with paler under surface, K+ yellow cortex, pseudoparenchymatous upper cortex (cell like structure formed by hyphae fusing together)

Getting to species level:
Lobes lacking marginal cilia (eliminating tenella and adscendens)
Soredia (fungal hyphae wrapped around a photobiont cell) developing on the underside of the lobe tips.
Lower cortex also pseudoparenchymatous.

Probable Physcia tribacia

Thallus section of probable Physcia tribacia