It could be legal Op.
But how much reliance you place on it being legal depends on if you only declared the £600p.m outgoing or the entire £200k loan to the mortgage company.
If only the first, then no one has checked it for AML purposes. So it might or might not be OK - as I said previously there are valid everyday reasons why your Dad may have structured it this way. Although if only the repayment was declared, this may be mortgage fraud.
If the second, then it has been checked and the bank must have been fine with a second charge over the property (others have said this is unlikely, I don't know myself - if the bank take priority and there was a hefty deposit it is not impossible).
You talk of being very young at the time. How long ago was it? That will make a difference to the AML checks done I imagine, although they have been around a fair few decades now.
Also, I am a random on the Internet who only can guess at the arrangements or your Dad's situation, so what do I really know?!, but I've found the thread and people's financial beliefs very interesting. Take proper legal advice if you are convinced that money laundering was at play.
But the first thing I would do is a good chat to your Dad. You sound a bit scared of him, but he really might have done this above board and in your best interest. You are an adult now and can and should approach each other as equals - but take the heat out of the conversation - I do not advise even the merest hint of a suggestion that you think money laundering or that you resent repaying him. Just check his motivations for setting up the loan the way he did. Your Dad may speak more freely just with you and your sister, than in the company of your partners. Keep an open mind. Explain your situation - not asking for a handout, but how best to financially manage things and he may be incredibly helpful to you as it sounds (to me) as if he is good with money and that he loves you very much in his own way. Family dynamics are difficult and sometimes very difficult to outgrow, but set aside preconceived ideas and keep a really open mind. All best.