Are you talking about the number of different permitted routes for a ticket? Surely something like Penzance/Plymouth-Inverness/Thurso would win? There must be dozens.
Via London (via Westbury or Bristol), into Paddington or Waterloo or Victoria or London Bridge then out of London via Liverpool Street or Marylebone or St Pancras or Kings Cross or Euston or Moorgate. Or via Birmingham and Manchester or via Birmingham and Stoke or Chester or via Shrewsbury or via Birmingham, Leeds and Appleby, then via Glasgow or via Edinburgh then via Inverkeithing or not, then via Aberdeen or not.