The information given by HMRC is as follows:
Income taxable on the arising basis should be converted to sterling at the rate of exchange prevailing at the time when the income arose.
Income taxable on the remittance basis should be converted to sterling at the rate of exchange prevailing on the date of receipt in the UK.
Gains on foreign life insurance policies, life annuities or capital redemption policies should be calculated in the currency in which the policy or contract is denominated and the gain converted to sterling at the rate of exchange applicable at the time of the chargeable event.
If you are unsure of the exchange rate to be applied ask your Inland Revenue office or tax adviser, or go to our website http://www.inlandrevenue.gov.uk
There is a downloadable PDF file called SA106, if you can't find it, PM me with your email address and i'll email it.
HTH
Mark