If a company wishes to vary the rights attached to a specific class of shares, what is the standard requirement under the Companies Act 2006, assuming the Articles do not specify otherwise?

To protect shareholders holding a specific class of shares (e.g., preference shares), their rights cannot be varied without their consent. Section 630 of the Companies Act 2006 requires either written consent from 75% of the class by value, or a special resolution (75%) passed at a separate meeting of the holders of that class.