Capital gain and loss in same year, carry forward loss

I made 12000 gain in 2020-2021, and 16000 loss in the same year. After reading thread here, my understanding to the gain and loss rule is: £12000 gain is less than 12300 allowance. so no tax to pay. £16000 loss can be carried forward for future to use. If I have made £12400 gain instead, I can carry forward £15900, as £100 is used to offset the gain to £12300. Am I right? If I have filed tax return online at the time of 2021 for the tax year 2020-2021, can I still claim above loss now? If so, should I use SA108 in writing with supporting documents or something else? Thanks!

Hi,
Unfortunately, gains and losses do not work that way.  
You need to set the loss against the gain before you can apply the annual exempt allowance, which results in an overall loss of £4000.00.  
This loss of £4000 can be carried forward.  You are able to amend your 20/21 tax return up to 31 Janauary 2024.  
After that date the tax return cannot be amended and you would need to claim losses in writing.  
You have 4 years in total to claim losses.
Thank you.