OP Mine did this last year, also starting at 8.30
It's not gone well from many perspectives. You do literally wait for a call back.
I wasn't able to answer my phone once and got ticked off for missing an appointment I didn't know I had! Then got ticked off for not booking a medication review. I did try but it fell off their list.
Many people complained and they added the extra step of a text to say "you will receive a call on x date" with an AM or PM type slot.
I used to have access to an appointments diary where I could book the meds review slot and know I wasn't taking up any urgent same day appointments. But my GP said they all prefer it this way.
I will say that I messaged the day after having a nervous breakdown and they were on the phone within an hour (long history of depression and anxiety) so they are on the ball.
They have had to change the timing for online submission and it now starts at 6am and continues till they fill up, which is often by 8. The entire surgery was only available for emergencies in Nov and Dec. I couldn't get my routine review and called the last working day before Xmas as a matter of strategy, it worked.
Also, at one point, the online system also went mad and sent out messages saying prescriptions were refused when they'd been issued. Poor receptionists had a tough time with that.
I have moved and joined a new surgery and tbh I don't know how it works. I do have Benenden though so if the new surgery is crap that will help.