You may update the details of S455 (Director's Loan) under supplementary form CT600A.
You may include the supplementary form from the main form by selecting the Box 95 under page 2.
Please refer below screenshot for more help
Click here to watch an example video of adding a supplementary form to CT600
Click here to watch how to claim Group Relief through supplementary form
Click here to watch how to add S455 Tax (Loans to Participators)