Does a change in company policy affect my employment contract?

The company I work for have issued a new policy stating that it is now a non-uniform establisment and that all employees must stop wearing their uniform and must now wear their own clothes.

My contract states that the company will provide a uniform which must be worn at all times.

The company says they have not breached my contractre provision and wearing of uniform because the new policy states that it is now a non-uniform business and therefore the contract clause is not relevant any more.

Is this correct and do change of policies make null and void contract clauses?