British Isles The longest theoretical line of sight in the British Isles is 144 miles (232 km) from Merrick, in the southern uplands of Scotland, to Snowdon in North Wales. I have found no longer sightlines, and none were found in a study by topographic researcher Chris Jesty in the 1980's. A 1990's Guinness Book of Records published this superlative, but gave the distance as 144 km (sic). Unlike the longer US views above, the line of sight is low altitude and passes primarily over the sea, so Snowdon would only be observable from Merrick on an exceptionally clear day. No sightings have come to my attention.
Merrick would be practically impossible to observe from Snowdon, because of the very thin aperture it shows behind nearby Lamachan hill. To give an analogy: if a colleague and I were in neighbouring rooms, and I were at a desk but the colleague were looking through an empty keyhole, he would probably see my clearly, but I would not see him. The "keyhole" is Lamachan Hill, which is much closer to Merrick, so an observer on Merrick would see Snowdon much more easily than vice versa. Infact, Merrick would be impossible to observe from Snowdon other than with a telescope, and then only if there were a suitable contrast with Lamachan Hill (e.g. snow or sun on one but not the other). That is why Merrick is not shown on the Snowdon panorama.