Loss of investment in company shares - capital loss

Hello, In 2016 I have invested my savings in a private startup company I was working for. I subscribed £25k ordinary shares. The company went into administration in May 2022 and I lost all of my investment. I have never gain anything from the shares, neither deducted any taxes before. I am doing my 1st self assessment now and was wondering if there is anything I should do to claim that loss against my profitable income? As the company was dissolved in 2022 I do not think I am qualifying for the Negligible value claim. Capital loss is another thing I can do, but not sure how to make the calculation and what to do next for it to become allowable loss. Also how much time do I have to apply for any relief? If I am granted a relief of £25k, does it mean I will get tax refund from previous years? Thank you!

Hi

In the capital gains section, you should select either "listed shares and securities or unlisted shares and securities, depend on the shares you had.  
Click on the dropdown and select 'NVC', as well as declare the loss.  You should also provide supporting evidence of the NVC as an attachment, to support your claim.  You generally cannot carry losses back.  
There are some siuations where this is possible.  Have a look at the helpsheet for losses at HS227 Losses (2023).

Thank you