Whether you need to register with the Information Commissioner's Office (ICO) depends largely on whether your business or organisation processes personal data as defined by UK data protection laws. Most businesses, including sole traders, are required to register and pay a data protection fee, unless they qualify for an exemption.
Common activities that necessitate registration include employing staff, marketing, and handling customer data. If you are unsure about whether your activities fall under these criteria, the ICO provides a self-assessment tool on their website to help determine if you need to register. Failure to register when required can lead to enforcement actions and significant fines.