Which train routes are permitted when going from Great Britain to other parts of the UK?

I often get rail & sail tickets which begin in Great Britain and end in Northern Ireland. These tickets usually specify which boat crossing they are valid on, but I've never seen them specify which rail routes they're valid on.

I've tried to work out exactly what routes are allowed - the tickets aren't international travel (they're all within the UK), so it would seem that the standard routeing guide should apply. However, I've found that the pink pages only list routeing points for Great Britain, not Northern Ireland. There's no mention of how to apply the routeing guide to these journeys (though there is an easement for Belfast to Glasgow Central via Paisley Gimour Street).

An example of the type of thing I'm trying to figure out is whether, when going from London to Belfast via Cairnryan it is permissible to use the East Coast Mainline, or just the West Coast Mainline. (But I'm more interested in how you work this out than the actual answer).