Calculating capital gains tax on shares

Hi there, I have a query regarding calculating capital gains tax on shares - shares that are bought and sold in USD and require a GBP amount on the self assessment form. The company I use (Morgan Stanley) provide a currency conversation for the proceeds of shares sold on my documentation, which aligns with what is deposited in my bank account. The trouble I'm having though is understanding the currency conversation for the shares I bought at time of purchase. I bought many shares over a one year period, and Morgan Stanley helpfully lists out them individually and as a total - but in $. How do I accurately calculate the $ into GBP? The currency conversion rate is not included in any of my documentation - only the amount in USD. Thanks, 

Name removed admin .

Hi,
To work out UK capital gains liability on the disposal of the shares, means that you need to convert the acquisition cost , disposal costs and disposal value using an exchange rate in use at the time.  
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 at Exchange rates from HMRC in CSV and XML format and for older rates at 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.