Why the cynicism?
You wrote: 3 Like Home offered a simple way to reduce the cost of using data abroad, so obviously it had to go. This is cynical in the extreme. The single fact of the matter is when roaming off its 'home' network- while you can modify handset settings to connect to networks in order of choice (incl. the various 3 international networks), in most cases you cannot specifically exclude a handset from connecting to a much stronger signal from a different network.
The GSM Network Information broadcast by cells, complies with GSM standards- which specify a territorial code. Its not the case that '3' could broadcast the equivalence of multiple identical international SSIDs- locking a handset to its SSID, and implementing a defacto 'international ban' on roaming on other than its own network. Perhaps its a path they may explore in future- but as it stands its simply not open to them.......
Having been stung by accidentally wandering onto the wrong network- I'm a big fan of upfront fees when abroad- irrespective of what network one might happen to be on.
Perhaps its time to revisit the GSM specs?