Both profit and loss arising on disposal of an asset are not allowable gains and allowable expenses(due to subjectivity involved in calculating them), respectively as per the guidelines of HMRC. So, if we had shown(added) a profit on disposal in our P&L, then we will deduct it. And similarly if we had deducted a loss(and shown it as an expense) we will add it back to tax adjusted trading profits.
As Noah has stated the profit on a disposal of an asset is not a trading profit and therefore if included within the accounting profit needs to be deducted in the adjustment of profits exercise