Question about monthly and yearly currency exchange rates

HMRC has currency exchange rates for (i) monthly rate (ii) average yearly rate published on 31 Mar (iii) average yearly rate published on 31 Dec I bought and sold several overseas securities for several times in a tax year, and need to file Capital Gain Please advise which of the below, 1 or 2, is correct? 1. I can choose only one of the (i), (ii) or (iii) above to calculate the purchase prices and disposal prices for a tax year, OR 2. I can mix and use any of the (i), (ii) or (iii) above for each individual purchase price and disposal price in the same tax year. (i.e. use (i) for calculating the purchase price of one security, and (ii) for calculating the selling price of the same security, when reporting CG in the same tax year.) Also, is there any guidelines for using the (i) and (ii) and (iii) above for calculating and reporting CG ? Thanks.

Hi,
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
Exchange rates from HMRC in CSV and XML format.
You are free to use any of the supplied rates or one of your own choosing. 
Thank you.