You can’t book an appointment online if you already have an application in. You will just get turned away and probably won’t get your money back. If you applied more than 6 weeks ago and travel within 2 weeks you need to ring the helpline and put an upgrade request in. You may or may not be lucky and it gets actioned. If it doesn’t you need to go to your local passport office 48 hours before travel. If it’s been less than 10 weeks you’ll have to pay a fee though. As long as everything is in order and you have proof of travel there shouldn’t be an issue of you getting your passport. Just be prepared for a last minute trip to the office though!