SDLTM33220 | Ordinary partnership transactions: Responsible partners - Para6
From HM Revenue & Customs · Stamp Duty Land Tax Manual
States that anything required or authorised to be done by or in relation to the purchaser under the transaction is required or authorised to be done by or in relation to all the responsible partners.
Subject to the provisions relating to representative partners below, the responsible partners in relation to a transaction are—
the persons who are partners at the effective date of the transaction, and
any person who becomes a member of the partnership after the effective date of the transaction.