How to work out Tax on Shares bought years ago before selling? Trading212

I have held shares in a company on Trading212 for about 6 years. Since then I have a huge amount of returns and now that it is time for my remortgage I'm wondering if it's worth selling some/all to pay down the mortgage now that my rate is going to be 4-5 times more.

Before doing that though, I'd want to know how much tax might end up being paid on my profits if I were to sell some. I know that it's £3k worth of profit is tax free.

But the shares were bought in a number of stages, so it's not like I have one price that I bought at and one price that I sold at. I have X shares at £y, Z shares at £A and B shares at £C etc.