UK residents with foreign income or gains: dividends: Underlying tax - insurance companies
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Finance Act 2000 made many changes to double taxation relief for insurance companies.
These rules are now in TIOPA10/S96 - S104 but they apply for accounting periods or periods of account (as appropriate) beginning on or after 1st April 2000.
Full details are given in the General Insurance Manual.