How to declare capital gains tax in a foreign currency

How do I declare CGT in a foreign currency held in a US fund within 2023-24? The tax-free allowance was £6,000 at the time and my total gains were just over £5,000. However, when I fill the amount in as a foreign dividend, the full amount is added to my taxable income. Do I even need to fill it in if it's below the tax-free threshold? Which boxes do I tick yes/no if I'm to receive the dividend tax free?

Hi,
You will need to work out the gain using pounds sterling in all parts of the calculation.  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: https://webarchive.nationalarchives.gov.uk/ukgwa/20231016190054/https://www.gov.uk/government/collections/exchange-rates-for-customs-and-vat and for older rates at:
https://webarchive.nationalarchives.gov.uk/ukgwa/20100202113554/http://www.hmrc.gov.uk/exrate/index.htm.
You are free to use any of the supplied rates or one of your own choosing.  
The gain is declared on SA108 (Self Assessment: Capital gains summary (SA108).  
Self assessment will apply the annual exempt allowance.
 If you were required to pay a foreign capital gains tax, you also complete SA106 (Self Assessment: Foreign (SA106) where you delcare the tax paid and claim a foreign tax credit of up to 100% of the tax paid.
Thank you.