Confidence: It's likely to be this, but I can't be certain.
Notes: Ah, now photographs of dry material have been posted, we can see that it is indeed very thinly grey-tomentose on the upper surface. This takes it away from the possibility of being P. degenii, so my first suggestion is incorrect. Always the danger of trying to identify a Peltigera from a photograph when the surface cannot be checked under a strong lens or stereo microscope. The rhizines are also confirmed as single, not tufted as in P. canina (which in any case is much more persistently and strongly pubescent and with a wrinkled upper surface). My point about the large apothecia now seemed to be answered, as there is a good match to P. ponojensis in Vitikainen's Europaean monograph, and this species has just such large, more horizontally held apothecia. However, Vitikainen pointed out there was problematic material in northern Europe with veins and rhizines that darken, and this is the case in the material posted here. The more recent account by Vitikainen in the Nordic Lichen Flora refers such deviating material to his own species, P. monticola, originally described from the Austrian alps. I think that in view of the general resemblence to P. ponojensis, but with very thin tomentum, and the underside with flattened veins that are white at first but darken, and single rhizines that are white at first but also become dark brown, the photographs posted here fit P. monticola very well. It is now known to have a disjunct, circumpolar distribution, coming south in the Alps, and Vitikainen notes that it is "Probably much overlooked". It is not a British species, and not one I have seen, so I cannot provide the ID with complete confidence, but I have very little doubt - and it doesn't match anything else!