gbrtr - UK Tourist and Scenic Routes (si404)
gbnb - Great Britain B Roads (si404)
What's the correct system prefix, gbn or gbr?
gbnb.
The country code is GBR but the system names are gbnm, gbnam, gbna and gbnb.
Yes, the gbnm, gbnam, gbna and gbnb systems are Great Britain specific, rather than UK-wide, with Northern Irish specific separate systems nirm, niram, nira, nirb due to the separate numbering system. If we have a gbrb system, it would have to include the Northern Irish roads in that, which is just silly as they are clearly a separate system with it's own numbering system (though what precisely that system is is a mystery as it seems pretty random).
Only gbrtr is using gbr and the system name is UK instead of Great Britain!?
Because it contains non-British routes and is a UK-wide system...