Sorry, but I am looking at the whole picture. I was only addressing the comment
Why are none of you querying why he wanted to shift a large sum to his kids?
Many people try and DIY contracts and ignorant of what they can and cannot do put in unenforceable terms. Heck, even trained professionals will accidentally put unenforceable terms into contracts. In turn, people often misunderstand a contract’s terms and think it prevents them from x, y, and z when it doesn’t.
It’s more likely to be a mixture of a poorly written DIY contract and misunderstandings than a diabolical iron clad tax dodge.
There are strings- he's calling it his income. It's not, so that's not truthful.
It is a type of income. That is truthful. Principle repayments are nontaxable income, while any interest OP is paying in her repayments is taxable interest income.
It’s not an IHT dodge, because according to HMRC, if he dies before it is paid off, the outstanding loan is still part of his estate for IHT calculations although the limit is the original loan amount (really only helps if interest is being charged). The rules for family loans are not the same as a commercial loan.
He won't allow it to be repaid as a lump sum.
Ok? This sounds a bit self sacrificing, a lump sum could be spent on an annuity that pays more than the monthly OP is resentfully paying him.