Looks like the non-Uk card thing might be part of the “new and improved” website. Here’s the written response I got from my enquiry:
“We have worked very hard with our suppliers to create an online booking system which is both customer friendly as well as compatible with existing rail systems. Unfortunately, at the present time we are unable to offer bookings through our website for overseas customers but this can be done via our agency for selling tickets overseas (www.britrail.com). I can advise that the Bargain Berth fare is a promotion only available through the ScotRail website and is not available through BritRail.”
Sounds pretty definitive – pity they aren’t clearer about it on their website, which I’ve been finding pretty frustrating to use in general.