IFM12148 | Offshore Funds: Overview of the offshore fund rules: commencement
From HM Revenue & Customs · Investment Funds Manual
The 2009 regime has effect for the purposes of -
income tax for the tax year 2009-10 and subsequent tax years, and for distributions made on or after 1st December 2009; and
for the purposes of corporation tax on income, for accounting periods ending on or after 1st December 2009.
There are some transitional rules for offshore funds relating to periods of account straddling 1 December 2009 and immediately thereafter - see IFM12150.