Hi @DieCryHate,
I'm so sorry you're going through this. You'll find good company on this board.
I also had an empty gestational sac although I was nearly 8 weeks, and mine was smaller than yours at the initial scan (mean diameter 15 mm). I did a lot of research because I felt nobody was being honest with me about the high likelihood it was a miscarriage - both doctors I saw insisted "we can't say anything for sure until you re-scan next week." When I re-scanned, my sac had grown to a mean of 23 mm and still no yolk sac or embryo, so I opted for the surgical management. This was about a month ago now, and while it was a really rough time, I am doing better now.
I will share some of what I learned during my week of (somewhat obsessive) research, and I have to say that based on this information you should probably be prepared for a negative outcome - I am really sorry if this is in fact the case, and in reality there absolutely are "miracle" outcomes that happen from time to time, but they are a very small minority.
From "Guidelines for diagnosing miscarriage" by the Institute of Obstetricians & Gynaecologists in Ireland: "The yolk sac is the first structure often seen within the gestational sac and it confirms an intrauterine pregnancy. The yolk sac is first seen by transvaginal ultrasound when the mean gestational sac diameter is greater than 5 mm, and should always be visualised when the mean gestational sac diameter is greater than 7 mm."
According to this same paper, "A diagnosis of pregnancy loss may also be made if the mean gestational sac diameter exceeds 20 mm in the absence of a yolk sac or embryo."
The cut-off for diagnosis varies between countries and some doctors are even more cautious than others - as you have said in the UK they tend to have the cut-off at 25 mm - but with a mean diameter of 23 mm I would imagine there is very little hope of a positive outcome. A mean diameter of 15 mm is enough to be "indicative" but not diagnostic of a miscarriage. An unusually-shaped gestational sac is also indicative of a miscarriage unless there is a specific reason the sac might show up irregularly due to fibroids, etc.
The reason they keep us waiting in agony is to reduce the very, very small chance that a healthy embryo might accidentally have been missed due to issues with scanning, as they don't want to risk accidental termination of a wanted pregnancy, and by waiting a week they can be 100% certain.
I hope this is helpful in some way although I am sure it is not what you want to hear. I just remember being desperate for information so I wasn't hanging on to false hope.
Please let me know if you would like any other information about the research I investigated or if you have questions that are less practical in in nature as well!