I thought I'd start a thread on unreachable trigs: maybe some pooled knowledge will result in progress.

England, there are several which are just awkward and needing luck or permission.
The only show-stoppers I can think of are Fort Borstal: I met the lady: "This is our home and we don't allow people in"; Fort Rowner may be unfeasible too - needs a letter to base commander. But at least the latter is visible.

Scotland, the stopper seems to be Mam Mor; others I can think of are Eastriggs and Kinniny Braes. I've only been (near) to the first of these.

Rob