@Frederick ********
you can book up to 30 days early I believe. use the link I sent you and see if you can book the appointment for the date you want.
Also there is NO advantage booking an appointment for right when they open (8:30) because you'll get caught in the "herd" trying to get in. Same for booking one when they re-open from the lunch closure (13:00). I'd say in the morning book it for 9:30 or 10 and in the afternoon, book it after 13:30..