Since you mention "the British Isles and/or Western Europe" I´ll venture a brave advance and throw in my hat for Germany. Getting around by rail is much easier than in the UK - mind you, our rail system is far from perfect as well, but it´s certainly way better than the British one. I´m not aware of any passes, and travelling by rail is not cheap but it´s fast and efficient. Tour the Rhine with its castles and/or Neuschwanstein - in other words, the full kitsch monty every Japanese tourist lusts for.
P.S.: And if you prefer a rental car over the rail - feel like doing 130 mph on the Autobahn?