SDLT is charged on completion or “substantially performed” land transactions.
Consideration is money/money’s worth (usually includes VAT); if contingent/ascertainable -treat as payable [though there may be an option to postpone tax payment].
Also note gifts for no consideration are not subject no SDLT
The rate of stamp duty you pay depends on what price threshold your property falls into and where the property is – there are different rates in Scotland and Wales compared to England and Northern Ireland. It also depends on whether you qualify for stamp duty relief as a first-time buyer or whether you have to pay a surcharge because you are purchasing a buy-to-let or second home.