what would be the date of your application - submission online
Date of application is the date you submit and pay online.
Or date of super premium appointment?
There is NO super priority at the moment. The service is suspended. Also note that the UKCVAS biomteric appointment is NOT a "super premium appointment'. It is only a biomteric enrollment appointment. Super priority is a UKVI service, when available and is NOT a UKCVAS service
Are they difficult to book in London and at what stage you book one during application process?
The appointment and super priority service are TWO different things completely. When super priority is available, it is part of the options in the online form. Your UKCVAS biomteric appointment can only bee booked AFTER you have submitted and paid online.
Can you change the date?
You can change the biometric appointment date if you wish. It has no effect on the UKVI ILR processing.