So.. back on track...
HMRC will go after you if you under pay your taxes. If you cooperate you will get a substantial discount on the penalty. Our tax rules (which are pretty much the same as other countries) require you to determine the correct amount of tax to pay under threat of prosecution if you get it wrong. Its a bit like being done for shop lifting, if a shop doesn't put prices on things and you pay less than they were asking.. Or have a very complex book requiring an expert to decode the prices. Its doesn't sit right in a world where they could pull all the information they have anyway (bank accounts, pay slips, investments etc) and ask you to validate it.
However, You can't expect to suddenly get 85% of your income because some "advisor" says its a loan, when it isn't. The individuals owe the money and HMRC will give them time to pay if they cooperate.