Indeed, on the British sleeper routes, you no longer have the choice to travel communally - i.e. the railway company won’t ever book you into a cabin with a stranger.
At least when I was doing the interrail those options were not available with the ticket without paying considerably more. I've taken 2 person cabins here to Lapland (from Helsinki, Finland) and they exhibit the problems in my original comment. And of course, if you're going to go solo in a 2 person cabin then you'll have to cover the cost of the other person.