The longest line of sight ever taken was from the Pyrenees as well, on a mountain called 'Pic de Finestrelles' 20-30 miles west of Andorra. The photo captured Pic Gaspard some 440km+ away in the Alps thanks to the right atmospheric conditions.
443 KM | Finestrelles, Pyrenees – Pic Gaspard, Alps
Current Guinness World Record Longest Line of Sight A thin line of Mountains rises over the Horizon: The Alps, with Barre des Écrins (4.102 m) in the middle left of the image. Distance443 Kilometer…beyondrange.wordpress.com
I feel like the conditions have to be precise to get it. Much like when I first posted in this thread, sometimes there is a slim or very wide window, depending on the conditions. It was indeed very sunny during Summer of 2022 when I crossed the Pennines and took that photo. But sometimes it can change with the snap of a finger. Hope you have another try!
Was going to post about another long sight I saw but I've ran out of time and will be back in a sec...
Interesting, I've actually been very, very close to Pic de Finestrelles, possibly even to its summit. Certainly did a walk along the summit ridge somewhere above Nuria, just to the south. Despite pushing 3000m, the peaks in this area are very non-technical and there are no more hazards than there are in the Lake District (excepting much more chance of severe storms).
It was July, but there was low cloud, so no chance of such views then. Even still, interesting that I was in the area which has the world's longest recorded line of sight!