Reporting US capital gains

Hello, I am a US citizen who lives and works in the UK. Last year, my US grandmother passed away and left me a sizeable US investment portfolio. I am now trying to report the capital gains of this investment portfolio in the last year. When reporting my capital gains, I have been converting USD to GBP by using the published exchange rates listed for the dates the transactions took place. However, because some of these investments are quite old (oldest from 2004), the exchange rate has changed quite a bit between then and now, and it is resulting in discrepancies between my gains and losses in USD and the resulting gains and losses in GBP. I want to claim an allowable loss, because I did not make a net profit on my investments this year. However, when I convert everything into GBP, and subtract the original share price from the amount they sold for, the calculations indicate that I made a small net profit, because the exchange rate is more favourable for USD than it used to be. This is not an accurate portrayal of my US finances - what should I do? Can I still claim an allowable loss? Should I do my conversions and calculations differently? Thank you for your help!

Hi,
For UK Capital Gains Tax purposes, you would use the market value of the shares at the date you inherited them and then convert to GBP sterling. The market value would be deducted from the disposal value of the shares. This will allow you to work out the gain or loss arising from the disposal.  
Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal. 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.