I have been all over the place on this, and I would probably say if you are unsure of the hotel give them a phone call. You might need to speak to a supervisor. Some hotels do have odd shaped/ larger rooms within categories that can do this, some are more uniform and there simply is not space. Or, book a king bed or double double and share?
London is a bit trickier due to room sizes/ space. The hotel name might help- message me if I can help. As a gold you will probably get a room upgrade, which should mean a bigger room, possibly space for a rollaway- but don't expect suites.
I have had hotels where they have been brilliant about adding an extra bed at no charge, and some where they just wont or want to charge silly money for it. Prague is not the easiest with this- depending who you talk to. I had a conversation where a rollaway was an extra 50 euro a night (for a child, not an adult). So, I said I would book a junior suite, but was then told I would still be charged the rollaway fee?? So booked a family room with sofa, and no extra fee- but surely they would only open out the sofa in the junior suite? Who knows.
However, I would say generally I have found Hilton hotels very accommodating and very good, often leaving a little something for the small ones. Most reception staff help if they can.