A scan in a week should give you a definitive answer but you are completely right - expecting the worst is the way to go. The amount of time they ask you to wait depends on the diameter of the gestational sac but in the same situation as you, I 'only' had to wait a week and then had surgical management the very next day. I already knew the pregnancy was doomed as I'd had beta hcg values that weren't doubling and started to decline.
If you're finding the waiting extremely hard, I would consider ringing up and asking to be seen after a week. The guidelines state the following:
1.4.9 If the mean gestational sac diameter is less than 25.0 mm with a transvaginal ultrasound scan and there is no visible fetal pole, perform a second scan a minimum of 7 days after the first before making a diagnosis. Further scans may be needed before a diagnosis can be made.
1.4.10 If the mean gestational sac diameter is 25.0 mm or more using a transvaginal ultrasound scan and there is no visible fetal pole:
seek a second opinion on the viability of the pregnancy and/or
perform a second scan a minimum of 7 days after the first before making a diagnosis.
The guidelines can be found at www.nice.org.uk/guidance/CG154/chapter/1-Recommendations#diagnosis-of-viable-intrauterine-pregnancy-and-of-ectopic-pregnancy
Some change should have occurred over a 7 day period of time.
As regards treatment, they cannot refuse you management and force you to wait it out. The guidelines say (relevant bit bolded):
1.5.2 Use expectant management for 7–14 days as the first-line management strategy for women with a confirmed diagnosis of miscarriage. Explore management options other than expectant management if:
the woman is at increased risk of haemorrhage (for example, she is in the late first trimester) or
she has previous adverse and/or traumatic experience associated with pregnancy (for example, stillbirth, miscarriage or antepartum haemorrhage) or
she is at increased risk from the effects of haemorrhage (for example, if she has coagulopathies or is unable to have a blood transfusion) or
there is evidence of infection.
1.5.3 Offer medical management to women with a confirmed diagnosis of miscarriage if expectant management is not acceptable to the woman.
1.5.4 Explain what expectant management involves and that most women will need no further treatment. Also provide women with oral and written information about further treatment options.
1.5.5 Give all women undergoing expectant management of miscarriage oral and written information about what to expect throughout the process, advice on pain relief and where and when to get help in an emergency[3].
1.5.6 If the resolution of bleeding and pain indicate that the miscarriage has completed during 7–14 days of expectant management, advise the woman to take a urine pregnancy test after 3 weeks, and to return for individualised care if it is positive.
I'm so sorry this is happening to you. I've just had my 4th loss and it doesn't get any easier but the shock is much less great 