You can learn more about this here - https://www.gov.uk/guidance/how-to-get-an-agent-code-for-corporation-tax-or-self-assessment
How do I get an agent code with HMRC to submit tax and accounts for my clients?
You can learn more about this here - https://www.gov.uk/guidance/how-to-get-an-agent-code-for-corporation-tax-or-self-assessment