If you've got old crypto gains you never declared to HMRC, you can still sort it (and coming forward first is way cheaper)
There's a lot of quiet panic about this so here's the calm version. Not declaring isn't the same as hiding, and it's fixable.
HMRC has a specific route for exactly this, the tell HMRC about unpaid tax on cryptoassets service. The key thing: if you come forward yourself before HMRC contacts you, the penalty for an honest mistake can be as low as nil. If they find it first, it starts at a chunk of the tax owed. Big difference, and the timing is the part you actually control.
Also worth knowing: from this year UK exchanges report your data straight to HMRC (the CARF rules), so "they'll never know" isn't really a thing anymore.
The actual move is just: work out what you really owe for each year, then disclose it. Most people owe less than they fear once losses and the allowance are in. Happy to point anyone to the HMRC pages if it helps.