Interesting subject. It would probably help to be on a big western hill with far-ranging views out to the Hebrides (or Ireland) in one direction. Cruachan is a candidate in those terms, and from Jonathan de Ferranti's excellent site,
http://www.viewfinderpanoramas.org/panoramas.html, it appears that it's possible to see Hecla on South Uist at 101 miles and various Gallowayish things (including Corserine and Merrick, both at 93 miles) in roughly the opposite direction. I've been on Merrick on a very clear day (5 Sept 1992) and could see Cruachan, and could also see Ireland (although Slieve Donard is "only" 88 miles away), but I didn't think to try and pick out North Wales, something I now regret.
Another candidate could be Torridon, as Conachair on St Kilda is visible from Beinn Eighe at 118 miles. Jonathan de F doesn't have a panorama for Beinn Eighe but does have one for Alligin. This doesn't include St Kilda but does have Boreray (but not Conachair) at 110 miles and in the south-east the moorlandy hills of Moy at 65 miles.
I climbed Beinn Edra on the Trotternish ridge on possibly the clearest/sharpest day I've ever been out (19 Oct 1999), and both main parts of St Kilda could be distinctly seen with the naked eye - Conachair is 87 miles from there. It was equally clear in the other direction and a huge stretch of the western seaboard could be picked out. I can't remember much detail but we could see Foinaven (77 miles) and I'm pretty sure we reckoned we could see Cape Wrath or somewhere close to it. On the southern chunk of mainland various big Lochaber things were in view and we had a go at trying to work out if we could see Cruachan (which doesn't seem to be quite visible from there according to JdeF's site). Cruachan is a good candidate for this kind of stuff generally, eg it has views to Mingulay and the Lammermuirs in opposite directions, 99 miles each time.
(This is all linked to the old game - which I've seen/heard discussed a time or two, of trying to jump from the north coast of Scotland to the south coast of England in as few views as possible - both theoretically and in practice.)