I too have been informed of the upgrade service when I rang to see about my options the other day. The lady I spoke to said you have to pay the difference to upgrade to fast track and can only apply if you can prove you are travelling within 2 weeks. My question was I’ve looked at fast track appointments at my local office and there aren’t any in the next few weeks so would this guarantee an appointment? She wasn’t sure about that but I assume if they offer the service they must put some spaces on reserve! However after a very pleasant call my application was approved later that day! It may have been a coincidence. Hope that helps!