Hi,
If you are a UK tax resident and you dispose of foreign assets, then you must declare the disposal in a Self Assessment tax return. If you dispose of foreign assets for more than you acquired them for, then you have a gain, which is taxable in the UK. You will need to calculate the pound sterling value of the assets at the time of acquisition and disposal, using a just and reasonable exchange rate in use at the times in question. For your convenience, there are exchange rates here:
Exchange rates from HMRC in CSV and XML format
and for older rates here:
Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009
You are free to use any of the supplied rates or one of your own choosing.
Thank you.