When we moved from Somerset to The Wirral, we were on the sea front at Hoylake. Even at sea level there were some notable sights. With binoculars, the Pennines, and the Winter Hill transmitter mast, were commonly there. The upper part of Blackpool Tower came out of the sea. The other way, Anglesey was visible, and Snowdon would have been were it not cut off by the closer Clwyd Hills. About once a year in summer, the Lake District appeared, Black Fell behind Ulverston. It was quite prominent so never understood why it appeared so infrequently. Neighbour said they had seen the Isle of Man a couple of times.
We might discuss longest line of sight of trains. Just round the corner, in West Kirby, in the mid-1960s just before the end of steam, on a still night from the sea front you could just hear and see trains on the North Wales main line, moving lights through binoculars if you knew where to look. There was a whistle as they approached Point of Ayr.
Back in Somerset, on the hill west of Minehead, Selworthy Beacon, I've seen with binoculars the Severn Bridge, and aircraft landing at Cardiff Airport.
Having done the Three Peaks (Ben Nevis, Scafell Pike, Snowdon) a couple of times, from both Ben Nevis and Scafell Pike I've seen Northern Ireland; I wonder if there's a point there where you can see both. Scafell Pike to Snowdon is more straightforward. It makes the huge distance covered between them seem inconsequential.