Section 27 | Time of disposal and acquisition where asset disposed of under contract.
From legislation.gov.uk
(1)Where an asset is disposed of and acquired under a contract the time at which the disposal and acquisition is made is the time the contract is made (and not, if different, the time at which the asset is conveyed or transferred).This subsection has effect subject to section 20(2) above, and subsection (2) below.
(2)If the contract is conditional (and in particular if it is conditional on the exercise of an option) the time at which the disposal and acquisition is made is the time when the condition is satisfied.