Thinking about non London OA routes, would the below route massively fail any abstraction tests?
Glasgow Central - Norwich (via Dumfries, Preston, Doncaster, and Lincoln Central), calling Paisley Gilmour Street*, Kilwinning*, Troon*, Prestwick Town*, Mauchline (if reopened - would require platform on the Annbank line side), Dumfries, Carlisle, Lancaster, Preston, Blackburn, Burnley Manchester Road, Hebden Bridge, Elland (if reopened), Brighouse, Mirfield for Transpennine Route, Wakefield Kirkgate, Doncaster, Gainsborough Lea Road (depending upon the platforms), Lincoln Central, Spalding for Boston and Kings Lynn bus link, Peterborough, March for Wisbech bus link, Thetford, and Norwich. *Pick up only from Glasgow, set down only to Glasgow.
Obviously, this would have to fit somewhere between Gretna Jct - Preston, any freight trains Doncaster - Peterborough via Lincoln, and the junctions at Ely. Also, the above route is by no means the shortest by any stretch of the imagination at all.
The route via Paisley, Troon, and the shortcut via Annbank is due to the single track sections between Barrhead and Kilmarnock, otherwise it could have been sent that way.
New links would be created between Ayrshire and North West Region, Carlisle, Lancaster, Preston, Blackburn, and Burnley to Wakefield, Doncaster, Lincoln, and Anglia Region, Lincoln to Norwich, and other overlapping links too.
A suggested frequency could be every 4 hours with departures from the end points and the middle at say 08:00, 12:00, 16:00, and 20:00. I admit that I have not worked out exactly how to fit this hypothetical service around existing trains, nor have identified a starting point to build around.