Blackwater - East Croydon is a fun one. The obvious and quickest route via Redhill (1h15m+) is not permitted.
If travelling in the peaks, the only through ticket itinerary reported by NRE for journeys to Croydon Stations is to West Croydon.
Outbound this can be done in a reasonable 1h20m with a change at Guildford, however the return itinerary takes 1h51m with changes at Epsom and Guildford.
It seems there is a god (or at least someone inside the industry who is taking note of this thread).
I searched this route today on a whim and, lo and behold, both NRE and GWR's mixing deck now report that the quickest route via Redhill is now permitted.
Thank you!