The site HeyWhatsThat.com is a pretty neat tool for answering this sort of question.
I've taken the liberty of creating a map showing the view from Lands End, which should be available to everyone at that link.
Here's the computer-generated panorama:
The Isles of Scilly are barely visible above the horizon (the horizontal magenta line) to the west-southwest, in the direction of the brown line.
The site also calculates the entire "viewshed" from Lands End and displays it on a map:
As can be seen from the pink shading on the Isles, their peaks can be seen from Lands End.
However, it is not possible to see from the cliffs at Lands End to the beaches of the Isles.
travel.stackexchange.com/users/31165/Michael Seifert
They can be seen from Cornwall, and photographic evidence isn't hard to find.
Isles of Scilly from Gwennap Head .
Source : Bob Jones via Wikimedia Commons.
travel.stackexchange.com/users/47776/Pere
I saw the islands from near St Just last week, a distance of almost 30 miles.
Very faint, but definitely visible.
I had no trouble with binoculars separating the islands and the hill on Samson.
travel.stackexchange.com/users/81809/Kevin King
I saw them using binoculars aboard the Brittany Ferry service from Plymouth, Devon to Roscoff, Brittany on 25th August 2018.
Halangy Down Transmitter on St Marys was visible.
The visibility was excellent, but perhaps there was a temperature inversion happening, or something like it, since I wouldn't have expected to be able to see over such a great distance (approx 90 - 95 miles).
I have seen the Lizard peninsula from Rame Head before with the naked eye, which is about 45 - 50 miles, and that was unusual, so I was really excited to see the Isles of Scilly.
travel.stackexchange.com/users/83861/Will Tall
Based on Mark Mayos excellent answer to this question and calculating the height 80 metres close to the Cornwall coast line you can only see a maximum of 32km with the naked eye.
This is based on the calculation: where d = distance in km and h = height in metres.
travel.stackexchange.com/users/19694/davidb
On a clear day is it possible to see the Isles of Scilly from Cornwall with the aid of binoculars? A distance of around 40km.   I know on a very clear day you can only see maximum 20km from sea level but the coast of Cornwall is higher than sea level. So does the height advantage of the cliffs enable you to see further across the horizon?  There are a couple of online forums that suggest you can see the Isle of Scilly from Cornwall but no photographic evidence.
Can you see the Isles of Scilly from Cornwall?
travel.stackexchange.com
<uk><islands><vantage-points><viewpoint>
travel.stackexchange.com/questions/104017
travel.stackexchange.com/users/19694/davidb
They can be seen faintly from Cornwall, but the beaches of the islands are not visible from the cliffs at Land's End. It is possible to see the Isles of Scilly from Cornwall, however it cannot be seen from the cliffs at Lands End. There is picture evidence to support that the Isles of Scilly can be seen from Cornwall though.
The isles of Scilly are barely visible above the horizon, but they are visible. There is even photographic evidence to rpove it.  it is not possible to see from the cliffs at Lands End to the beaches of the Isles.