Section 879P | Date of acquisition of relevant asset
From legislation.gov.uk
(1)A company that acquires a relevant asset in pursuance of an unconditional obligation under a contract is to be treated for the purposes of this Chapter as having acquired the asset on the date on which the company became subject to that obligation or (if later) the date on which that obligation became unconditional.
(2)An obligation is unconditional if it may not be varied or extinguished by the exercise of a right (whether under contract or otherwise).