The sponsors make Big Buck's 5-4 favourite to retain his title, following an impressive victory in the Long Distance Hurdle at Newbury last Saturday.
Diamond Harry is next best at 6-1 and Williams, who has just 16 horses in his care, said: "I really don't know what the long-term goal is, but the bookmakers make him a relatively short price for a race that is four months away.
"What we are hoping for is a natural progression and it should be a good race at Ascot."
Andy Stewart, who owns Big Buck's, is also relishing the prospect of the pre-Christmas cracker.
He said: "We're pleased with Big Buck's and the fact that a good horse like Diamond Harry is taking us on at Ascot can only be good for the sport."