VATREVCON36100 | How the construction reverse charge works: Partial exemption and the reverse charge: Partial exemption
From HM Revenue & Customs · VAT Reverse Charge for Building and Construction Services Manual
If a partially exempt business receives reverse charge supplies, they must still apportion the input tax claimable on the supply according to their partial exemption method; it is connected with the outputs it is used to make, not with the output tax accounted for under the reverse charge mechanism.