Liquidating BTC (UK)

So if I wanted to sell my BTC for fiat, either in need of an emergency or in 25years, whatever it may be, how does it work with tax?

I buy my BTC on a platform and transfer it to an external wallet, so if I'm correct, the bank knows where my money is going, so they have knowledge I have BTC through KYC?

If I go to sell this BTC and suddenly there is £££ in my bank account, do I have to declare this?

Would it be my job or someone else's job to look into if I've even made a profit from DCAing for years and therefore have to pay tax?

Any advice appreciated, Thanks