ARTG3110 | Reviews and appeals for indirect taxes: Appealing against a decision or assessment: Customer does not reply to the decision letter within the time limit

From HM Revenue & Customs · Appeals reviews and tribunals guidance

If the customer does not reply to the decision letter within the time limit, the matter may be considered closed.

But see ARTG3160 about late appeals and ARTG4300 about late acceptances of a review offer.