CTM80355 | Groups: group relief: overseas permanent establishment of UK resident company - meaning of attributable to overseas permanent establishment
From HM Revenue & Customs · Company Taxation Manual
For general guidance on the restriction of losses and other amounts that can be surrendered as group relief by a UK resident company that trades through an overseas permanent establishment see CTM80350.
For the purposes of the restriction, a loss, etc is attributable to the overseas permanent establishment insofar as:
looking solely at the activities of the permanent establishment, it would be surrenderable as group relief,
by following the principles for determining what loss or other amount could be surrendered by a non-resident company carrying on a trade in the UK through a UK permanent establishment.