There are certain races which at this stage are are a bit risky to bet on, especially the 3 novice hurdles (the Supreme, Ballymore, and Albert Bartlett) because trainers might not yet have decided which one to go for. The Champion Bumper is also a bit of a lottery. And in the handicaps there's not much point betting on a horse unless you know it will definitely get in the race.
But there are 28 races in all, so there are still plenty of opportunities to have an ante post bet.