One thing puzzles me: How come Rail Europe can do it and at reasonable prices when the operators themselves cannot do it? OK, I know they issue separate tickets per sector, but even so...
Rail Europe is part of SNCF, so it only does the full range of French rail tickets. For other countries, it mostly can only sell full-priced tickets, simply because its system is not aware of any others. If you want, e.g. a DB Spezial ticket, you won't find it at Rail Europe, you need to go to DB or maybe Loco2.
All in all, the current state of European cross-border rail ticketing gives a good idea of what UK rail ticketing would be like had British Rail been privatised the way the government of the time originally planned.