You can only have one active application at a time. So if you submit an online application you won’t be able to also book a fast track. You could try and wait it out to try and get an appointment but obviously the longer you leave it the less time there is to process a standard application if you can’t get a fast track. You’ll need to have had your application in at least 6 weeks before you can upgrade and travelling within 2 weeks. If I were you I’d submit a standard online application now then it gives you enough time to be able to upgrade if you need to. I used post office digital check and send. It took about 5 weeks in total from applying to receiving my passports. That was for a adult renewal with name change and 2 first child passports. Maybe that is a bit quicker as your application is checked before it’s sent so less likely to have issues and go through quicker.