This would normally be handled as a pre-tax adjustment. The depreciation is added back and the capital allowances / AIA is claimed on the CT600.
For general reporting and in order to get a rough tax calculation you can post this expenditure to a unique balance sheet code and flag that code as a “deduction” in the tax summary report (just click the blue cog wheel).
This will then deduct any balance posted to that nominal code from your taxable income.