The organisers should have a triplicate book of 'tickets' from the BSJA - the BSJA has the white top copy, the competitor has the second yellow copy and the show keeps the pink bottom copy. As far as I know it still costs £6 and yes you just turn up and pay the entry fee plus the extra for the ticket.
Are you going to Port Royal? If so entries on the day, buy your Ticket to Ride at the same time. They often offer them as prizes in unaffilliated and warm up classes.
I might be going on Sunday with a youngster for the warm up class and maybe BN - look out for a plain brown TB with a red ribbon in his tail being extremely nappy!