How does one pay tax on box profits? (UK)

Let’s say I sell all my BTC which I have accumulated little by little over many years. I have no idea how much I paid for all of it in fiat because we’re talking loads of tiny amounts over a long period of time, plus some swaps from different currencies etc. I know I’ll be liable for capital gains tax in the UK, but honestly have no idea how this would be calculated or paid? If I withdraw everything can I expect a letter in the post from HMRC? Or just hope it goes unnoticed and live in perpetual fear of receiving a massive bill in the future? Honestly have no clue. Hopefully I’m overthinking the whole thing. Does anyone UK based have experience / advice? Cheers!!