Past losses

What does "ref uk tax rules" mean?

What do you mean by past losses "going back up to 12 years"? As far as U.S. tax law is concerned, you only have a loss when you sell. Did you sell shares at a loss 12 years ago, or did you buy shares 12 years ago and sell at a loss more recently?

When you sold shares at a loss, were you a U.S. citizen, a resident alien, or a nonresident alien? Why are "uk tax rules" involved? We can help you with your U.S. tax return, but not with a U.K. tax return or any other country.

Did you file a U.S. tax return for the year or years that you sold shares at a loss? Did you report the sales on your U.S. tax return?

If you are a US person for income tax purposes (resident alien, citizen, or green card holder), you pay tax following US law only.

For investments like stocks, you only claim gains or losses when they are "realized" -- that means the loss or gain is made real by selling the investment back to cash or for another investment.  For example, suppose you bought $10,000 of stock in Disney at the $180 per share at the height of the pandemic, and it is now worth $90 a share.  You have lost on paper $5000, but that loss has not been realized, it;s only on paper.  If you sell the stock for $5000, then you have a $5000 loss.  But if you want to hold it and see if the price goes up, you have nothing to report as a gain or loss.

Likewise, if you bought stock in Tesla at $100 and it is now worth $400, you have a gain, but it is not taxable unless you sell or cash it out or convert it to another investment in some way that makes the gain "real."