If this question had related to the type of bonuses received by senior employees in major financial companies within the City of London, there might be an argument for stating that 'custom & practice' forms part of an employees contract. (People working in those types of jobs know that all of the big companies pay annual bonuses, based on the company's performance, which can sometimes run into hundreds of thousands of pounds. If an employer suddenly withdrew these bonuses, their employees could argue that there had always been an 'implied contract' that they should receive them).
However, ordinary Christmas bonuses (where they exist) are simply a gift from the employer. If your Aunt Edna decides not to give you a Christmas present (even though she's done so for the past 50 years), you've got no right to demand one from her. Exactly the same position applies when your employer decides not to give you a gift.
Chris