VATVAL03600 | Apportionment of monetary consideration: liabilities of supplies in relation to which an apportionment is made
From HM Revenue & Customs · VAT Valuation Manual
The final step before considering an apportionment proposal from a trader is to ensure that all the supplies to which a consideration relates have been identified and their respective liabilities have been established. You will then be in a position to consider the apportionment method itself.