5 minutes before post, you see your horse at 8-1, you have the right (in the UK) to bet it at that price or bet it at SP which means that whatever price the horse goes off at.
Fixed pricing is great because often you can bet a horse an hour before the race at 4-1 and then watch him get bet down to 7-4 and win for fun and you have a 4-1 winner. Of course, your 4-1 horse could go off 7-1 and you are stuck with 4-1, but if you follow the races closely, you can usually determine which horses will drop in price...
Wish they would do this in the States. They tried once at the defunct Sport of Kings and they got clobbered so that was that.