Mulder's couch had a large role in X-Files fanon, and, yes, it was always called a "couch" by American fans, so I assume from that that it's the usual American word. I did pick up quite a few American expressions at that time, since I was working so hard to ensure that my American characters sounded properly American, that quite a few of the expressions crept into my own spoken vocabulary. However, I'm 99.9 percent sure that "couch" wasn't one of them. My parents, I'm almost entirely sure, also call it a "couch".

I find it interesting how so many of our word choices are not conscious choices at all, but are just a case of us using the word our parents used back when we were two, and leaning our vocabulary. I guess that's the whole point of the U / non-U thing. You can acquire money and aim to mix in posh circles, but you can't shake off those unconscious vocabulary markers that brand you as being not properly posh.