I only mentioned visibility at sea level to make it clear the pollution wasn't at a very low level.
For what it's worth the cliff top at the Lizard was very clear from the coastal path. I recorded the visibility as VG that day. Whilst the horizon over the sea would agree with your figures, there's cliffs 300' nearby from which it is possible to estimate visibility even if you are at a fairly low level.
It is Interesting how far away you can see the Cornish cliff tops when you are out in a small boat, quite useful on occasion.
You might be interested in this photo taken from Scilly. The highest point of Scilly is 47m (approx). This was taken near 30m. The hill to the left is Chapel Carn Brea between Lands End & Cape Cornwall, >30 miles away.
I love the way you can only see the top half of Longships lighthouse.
There was a saying when I lived out at Sennen, 1 mile from Lands End , if you can see Scilly it's going to rain. The clearest visibility here is typically prior to a warm front, not in a cold sector. Salt spray often serious affect cold sector air out here on the Atlantic fringe.
To get a photo like that, quite clearly the visibility near sea level was E++ , certainly way >50miles. I have seen Lundy from the moors above St Ives.