Earlier quoted context omitted.
The problem is that there's nothing illegal about putting your money in secret swiss bank accounts per se. It first becomes tax evasion (in the UK anyway; details may vary) the moment you fail to report income or capital gains that are taxable, since the UK does not tax wealth.
Yeah, but you don't accidentally both place your money in secret Swiss bank accounts and fail to report it. You may want a secret account for privacy and you may forget to report all your money. Doing both at once, however, is highly unlikely. There's an end to plausible deniability and beyond that end should lie prison time.
Expect lengthy trials with all kinds of details showing up that creates sufficient doubt to make it hard to justify a prison sentence.
For starters, unless HMRC can prove that you have received the money in a way that should be taxed in the UK, they have no reason to expect you to report the money, and so most people don't.
E.g. for my part I'm Norwegian, but have been ordinarily resident in the UK for the last 13 years or so. The first year I lived here, I was not ordinarily resident for tax purposes, and maintained accounts in Norway that HMRC had no expectation of knowing anything about, and no legal basis for me to report.
But if I had had a large enough fortune and income, it'd have been trivial for me to maintain a status as not ordinarily resident much longer, and legally avoid reporting any income due from work done outside the UK at that point. The same is possible for UK citizens who move out of the UK to work for some time. For me there was no point, since all my income came from the UK, and so I brought my money into the UK once I'd tidied up my Norwegian affairs.
If you wish to defraud the HMRC, then, and create plausible deniability, you move out of the UK, or at least spend enough time outside the UK for a few years to be able to claim that when you receive that X million payment from WeAreTaxEvaders Ltd. in the Cayman Islands, directly to your secret account in Switzerland, it is income due to you entirely from work done outside the UK in a period where you were not resident in the UK for income tax purposes. Then you move back, and conveniently don't tell the HMRC because, you will claim, it has nothing at all to do with them (of course it is not at all because you don't want to risk that they take a closer look at the paper trails related to WeAreTaxEvaders Ltd. and maybe find traces of payments from the UK etc., or a timeline that doesn't match when you were abroad), and you way have had legitimate reasons to want to e.g. invest in Swiss shares or whatever at a later date when you retire to a cabin in the Swiss alps. Or whatever. If you are telling the truth about this, it is none of the HMRC's business that the money is sitting there unreported.
This is the problem with going after these schemes: There are any number of completely legal - for good reason, often, - mechanisms that allow the unscrupulous to create sufficient plausible deniability that actually proving a case becomes incredibly hard. It's easy to sit and assume that of course these are tax evaders. Except some non-trivial percentage of them are not.
HMRC risks not only not getting the actual tax evaders imprisoned, but not getting their hands on the money either, if they go to court on insufficient evidence. Going after just the money is a lot easier since people are often willing to pay to make the threat of further investigation go away even if they see the risk of prison as remote.