@happyjohn
My pleasure.
1) Fear of an offshore agent
Let's play a game.
I'm a UK tax collector, appointed to penetrate offshore tax evaders.
The first thing I'd do is find popular offshore service provider websites, the likes of SFM and OCC, and offer them a bit of cash in exchange for information:
"Hello, I'm a big brother from the UK. You have two choices:
1. Give us information about all UK citizens and residents who have used your services in the last 3 years. We'll pay you ~fifty thousand pounds for the administrative work. We will not disclose to 3rd parties from where we obtained the information.
2. Alternatively, we will hold you liable for promoting tax evasion and unwillingness to cooperate."
Would your offshore agent risk fines and shock imprisonment to protect you?
2) Greed of an offshore agent
Once the offshore agent has hoarded details of ~a thousand UK customers, he can approach the UK tax office anonymously with a business proposal. For example, offer to sell the details of potential tax evaders for a hundred thousand pounds.
Denmark paid a million for the well-organized details of Danish citizens/residents featured in the Panama Papers leak. It's a lucrative business opportunity, particularly if the offshore agent is thinking about retirement. Why not cash in on lifetime of work to make the retirement a little sunnier?
3) Accidental information leaks from the offshore agent
Incorporation windmills are not information security experts. The Panama Papers and Paradise Papers scandals are perfect pieces of evidence. Documents featuring sensitive information are probably printed out, on the table, under a coffee cup; digital files held on an unpatched Windows XP file server.
...
3 Major Risks, and we've not yet touched banks and payments, CRS/FATCA, or Bilateral Information Exchange Agreements. If there's interest, I can elaborate a little further.