You can close your self-employment status online through this link:
https://www.tax.service.gov.uk/shortforms/form/CeaseTrading
You will also need to send your final tax returns before the deadline.
And finally, please let us know that you are no longer self-employed, so that we stop sending you shift proposals and so we can proceed to closing your account on Brigad. Indeed, without a legal structure like a self-employed status or a limited company, you may not work with Brigad anymore.