Yeah, all the free ones were the first to get booked up. Happened when I was booking my ILR appointment, too. The 8:30am appointments would be the last to go. Personally, I think it's yet another part of the scam.
"Appointments are free*"
* Some restrictions apply. Must book an available appointment after 2pm but before 1pm in the same calendar day or an out of hours premium will be applied.