The ICO's advice is that:
"You are not required to automatically ‘repaper’ or refresh all existing DPA consents in preparation for the GDPR. But if you rely on individuals’ consent to process their data, make sure it will meet the GDPR standard on being specific, granular, clear, prominent, opt-in, properly documented and easily withdrawn. If not, alter your consent mechanisms and seek fresh GDPR-compliant consent, or find an alternative to consent."
