Change the capital gain tax calculation with different foreign currency exchange rate

Hi, I would like to ask whether I can change the capital gain calculation by using different exchange rate base; i.e. using daily foreign currency exchange rate to using HMRC yearly average exchange rate? Last year I employed a tax advisor to submit my self-assessment. The tax advisor used the daily foreign currency exchange rate. This year I would do the self-assessment by myself. I would like to use HMRC yearly average exchange rate to calculate the Capital Gain Tax. I am amending the last year's capital gain calculation with HMRC yearly average exchange rate. I will submit the amended last year self-assessment and this year self-assessment together. Then I should pay the total capital gain tax amount of last year and this year with yearly average exchange rate deducting the amount I paid last year with daily exchange rate. Is it correct?

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:

The National Archives: Exchange rates from HMRC in CSV and XML format

For older rates at:

The National Archives: 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.