HMRC have updated their Business Income Manual guidance at BIM 40095, which now reads as follows:
“Businesses that prepare accounts under IAS or FRS101 (BIM 31025) will recognise and measure foreign exchange gains or losses in accordance with IAS21 The Effects of Changes in Foreign Exchange Rates. Businesses using UK GAAP may have adopted FRS102 section 30 Foreign Currency Translation.
A full description of the treatment of foreign exchange gains and losses is in BIM 39500 onwards. There is also a Statement of Practice, SP2/02, that sets out HMRC practice with regard to the exchange gains and losses of unincorporated businesses.”
https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim40095