CTM90640 | Corporation tax self-assessment (CTSA): Claims and elections: Must be made in a return
From HM Revenue & Customs · Company Taxation Manual
A company cannot make certain claims before it delivers its return for the period to which the claim relates (FA98/SCH18/PARA9). These claims are:
repayments of IT under CTA09/S3
repayments of IT due to exemptions from IT under the CT Acts,
investment relief under the corporate venturing scheme.
There are exceptions to this requirement under FA98/SCH18/PARA9(2A)&(2B), which mirror those provided for at FA98/SCH18/PARA57(1B)&(1C), see CTM90625.