Complex sentence: At 17, he moved to London and became a successful butcher's assistant, but left his job at 18 to gamble on the horses at Ascot Racecourse in Berkshire.
Split into simpler sentences: At 17, he moved to London to become a butcher's assistant there. Although successful at his job, at 18 he gave it up with the plan of making money gambling on horses at Ascot Racecourse in Berkshire.