SDLTM04070 | Scope: How much is chargeable: Non-cash consideration: Services provided FA03/SCH4/PARA11
From HM Revenue & Customs · Stamp Duty Land Tax Manual
Where the purchaser provides services, other than works of construction, improvement or repair of a building or other structure, the chargeable consideration is the open market value of those services.
See SDLTM04140 for guidance on how open market value is to be calculated.