I'd say that a Denon AVR with Ethernet and RS-232 port is highly likely to be
generally compatible with
Cynical Denon - many of the mentioned "third party control devices" control it through the serial interface we use.
There is of course no guarantee, but while Denon doesn't care about breaking my plugin, they do care about breaking all that expensive "commercial" home automation integration that makes high-end installers specify their equipment.
Cynical Denon may not be able to cope with any
new functionality they may have added recently; my newest "Denon" is a Marantz NR1607, which is about 3 years old tech by now. I'm sure people will let me know if that happens.
But the existing functionality is probably fine.
The ultimate test would of course be finding someone who's tried an AVR-X2400 with Indigo. I doubt that there's any UK/USA distinction in that area.
Cheers
-- perry