SDLTM08100 | Scope: When is Stamp Duty Land Tax (SDLT) chargeable: Contracts and substantial performance: Definitions FA03/S44(10)
From HM Revenue & Customs · Stamp Duty Land Tax Manual
Completion means completion of the land transaction proposed, between the same parties, in substantial conformity with the contract.
Contract means any agreement.
Conveyance includes any instrument, whether in written or electronic form, including a lease.