Does a faint positive at around 15dpo mean it will be a miscarriage? Or is faint positive ok?

Sorry to hear about your miscarriage. The faintness of a line does not determine how strong your pregnancy is even though some women believe this.
It depends when you ovulate. Some women don’t even get a faint until after 15 so try not to worry about the line and wait for your blood test!

We don't get a blood test without asking until we're around 12 weeks I think. But they may test early due to previous mc

With my two live children my tests where very strong with my three miscarriages my test where very weak. But that’s just me. Some people don’t ever get strong lines. There’s no actual way of telling until you get a blood test. Positive thoughts and love sent to you