UK crypto tax query

Hi all. I have been in crypto for a good few years but did lots of trading and buying and selling coins on a few of the bigger sites.

I have some bitcoin offline on a hardware wallet but i cant even remember where i bought it or where it came from. i have dipped in an out if it over the years. most of the shit coins i bought tanked and i pretty much just gave up on them.

I never took any money back into fiat in my bank account, but also never did any tax return or paid any taxes. After or during this run i may consider taking out around £20k by selling some bitcoin.

My question is how would i pay taxes on this with such a long and complex crypto history, will i be required to try and work out all my previous trades and taxable events through the years?

Can you just assume a baseline and attempt to pay the max tax possible for amount being withdrawn now, Im assuming previous years of not paying tax will come back to bite me?

Ideally i would like to buy more bitcoin and dca for a while, but these tax worries about getting it back into fiat eventually are holding me back.