I’ve seen the Cheviots from Lochnagar – 111 miles apparently.
I’ve almost done the reverse on a few occasions. From the summit of the Cheviot up to the tops of Angus, Mount Keen and Glenshee, over west towards what I thought must be Ben Lomond and Lawers area, and south down the coast to Yorkshire and SW to the Lakes. I’d been told that you should theoretically be able to see Ben Macdui but this (other) cool panorama generator suggests that would instead be Braeriach – though good luck making it out amongst the mass of hills. Cheviot is a good top for long views in several directions. According to that site the view theoretically extends 210km into the Cairngorms, 110km into the North Sea, 159km into the N York moors, and 143km down to the lakes, taking quite a bit of landscape in between.
Slieve Donard suggested above by PZ_Steve looks pretty good for distant views in all directions though – 212km to the SE, 197km to the N, 162km W, 193km E (in theory).
Also RE seeing Norway from the Cairngorms – impossible, but I’ve been on the top of Beinn Mheadhoin or Ben Avon IIRC and seen what looked like mountains “floating” in the distance to the east, seemingly way beyond anything else so understand where the idea might come from. Someone later suggested it could be a result of the inversion and diffraction of the light or some such, but I can’t remember the specifics. Pretty wonderous stuff in any case.