penalty or tax due on shares not declared?

Hi, Sorry to bother you; I 'm a French national living in the UK and I'm resident there for tax purposes. In the 2020-2021 tax year, I had to sell the shares that I had at my bank in France in order to gain some liquidity as Covid was happening. I thought at the time (and to be honest, I'm pretty sure I was told on this forum as well that I didn't need to do anything but...) that, because I was below the 2020-2021 annual exempt amount at the time (£12300) for the capital gains tax (https://www.gov.uk/guidance/capital-gains-tax-rates-and-allowances), I didn't need to declare anything... But I was recently told otherwise that, because the assets were foreign, I shoud have declared them in the 'foreign' section even if I was below the annual exempt. My question is : could I be fined now and how much? as I understood, because I was below the thresold at the time, I should not have paid tax anyway (or maybe rules are different for foreign assets ?)...would I have to pay something now, was there any tax due on that income? Sorry, I'm a bit stressed about all this, Many thanks,

Hi,
As the disposal of your shares ocurred outside of the UK, then the disposal should have been declared in a self assessment tax return, even if the gain was below the annual exempt allowance (there in no deminimis limit for foreign income or capital gains).  
As it is now too late to amend your 2020 to 2021 tax return, you should write to H.M. Revenue and Customs Self Assessment BX9 1AS, notifying of the share disposal and the resulting gain.  
Provided the recalculation of your 2020 to 2021 tax liability to include the capital gain, will not result in futher a further tax liability (as below the AEA) no futher action would be taken.  
If further tax is payable, a Revenue Assessment would be raised and a charge placed on your self assessment record, which would be subject to late payment interest and penalties.
Thank you.