ARTG2220 | Reviews and appeals for direct taxes: Appealing against a decision: Customer does not reply to the decision within the time limits
From HM Revenue & Customs · Appeals reviews and tribunals guidance
If the customer does not reply to our formal decision notice, such as the notice of assessment, amendment, closure notice or determination, within 30 days, the time limit for accepting a review or appealing will have expired and in most cases the decision stands, see ARTG2200.
But see