OT42560 - Non-Residents Working on the UK Continental Shelf: Capital Allowances - Plant & machinery allowances - Foreign currency
The basic rule set out in CTA10\S5(1) is that a company should calculate its profits in sterling for the purposes of corporation tax unless a different calculation is required by CTA10\S6 - S10 and in particular CTA10\S9 for non UK resident companies preparing accounts in a currency other than sterling.
Where profits or losses of a business are calculated in a currency other than sterling, capital allowances on plant and machinery are also calculated in that currency. This is because capital allowances are deducted in computing the profit for CT purposes under CAA01\S2(1)(b) - see CFM64150.