CGT on 2020 gains, carry over of 2021 losses

Good day, Back in 2020, I had made a profit of £21,000 from buying and selling cryptos. Unfortunately, in 2021, I moved the amount to the crypto exchange FTX, and added an additional £18,000 in funds. Regrettably, on the FTX exchange I purchased a derivative, which moves 3 times as much as when the price of Bitcoin goes up or down. When Bitcoin dropped in price, this investment lost most of its value. As FTX was then declared bankrupt due to fraud, I was forced to liquidate what little I had left, making a near total loss of £39,000 in 2021. I made the mistake in thinking that I should pay tax on crypto gains when transferring the funds to my bank account, and hence I did not declare the profits on the £21,000 profit. As this is £9,000 above the 20/21 CGT allowance of £12,000, am I correct in thinking that I owe HMRC capital gains tax of approximately 20% £9,000 = £1,800? If this is the case, how do I go about in advising HMRC and paying this tax? This was a genuine error on my side, but I am wondering if I will receive a fine for this? I am working with an accountant to put together detailed records. Also, with regards to the loss of £39,000, is it possible to carry this loss forward and offset against any future gains? If so, how do I advise HMRC of this? What supporting evidence will I need to present? Many thanks

Hi,
You have 4 years from the end of the tax year a loss arises, to claim for losses any longer and it is too late, the losses will not be allowed.  
So for losses arising in 2020 to 2021, you have until 5 April 2025 to claim losses.  
You will need to do this in writing and post your claim to HM. Revenue and Customs Self Assessment BX9 1AS.  
When claiming losses or declaring a capital gain, you need to provide supporting evidence, such as a spreadsheet itemising gains and losses in the tax year, along with a calculation of the gains/losses.  
Losses that are allowed, can be carried forward indefinitely and set against a future capital gain.  
You should report your gain in writing to the same address.  As it is too late to amend your tax return, HMRC will manually calculate the amount of underpaid tax and raise a formal assessment using current legislation for this purpse.  
You will have 30 days to pay the tax from the date the tax is charged.  Penalties and interest will be applied against the outstanding tax charged by this assessment once the 30 days have passed.  
Interest is charged by law and is 2.5% above the Bank of England base rate.  There is a calculator to help estimate your penalties at:
Self Assessment tax returns
Thank you.

Thank you for your feedback. I am really concerned that I will get a fine/penalty. I did know that I had to pay taxes over the £12,3000 threshold, but I misunderstood what "realised gains" means -- that's to say, I thought that a person realised a gains when the funds were transferred to their bank account, because then you have realised those funds. I appreciate I misunderstood this and hence did not pay the required tax, but does this mean I will get a fine? Apprecaite you don't have all the details on my case, but any views would be helpful. Thanks

Hi,
It is likely that a fine will be imposed for late submission of the informatoin. Interest will also be charged for late payment.
Thank you.