I would have thought that the FPL is exactly the place to put all that stuff, if you filed an FPL for search and rescue purposes. If you don't turn up, they'll pull out your FPL and your precise routing is available.
In terms of communicating your route to enroute ATC, they probably don't have a copy of your FPL anyway. Use sensible reporting points. "Estimating abeam Swindon at..."