Sold 10BTC (at ~$10k each) for $100,000 back in 2018 on the way down from its ATH, back then, of $20k. Not paid tax question. Just sat in account.

So I sold 10 bitcoin which I had purchased back in 2011 for roughly $100,000. Since then the cash sat in my exchange account (a very reputable one). I know I've been dumb here, 1 for leaving it in an exchange, and 2, for not paying CGT on it. I thought back then that I would only pay tax once I had withdrawn to my bank account, I didn't know selling on the exchange was the taxable event. From the research I had done earlier during that time, I remember reading that cryptocurrency wasn't taxable, but that was probably misinformation or outdated.

I want to sort this out and pay the correct tax without putting myself in the shit. What is the best course of action here? I assume withdrawing that amount to my bank account is going to raise some alerts as it's way above any other previous deposit.