I would disagree with some of the things that you say.
"£200k to you means it's outside of his estate, wouldn't be taken into account for IHT"
That would be the situation if it really was a "gift". But here there is a paper trail that it is a loan and there are regular repayments. In this situation the £200k is still an asset of the father and so would be taken into account for IHT purposes.
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"You're then paying it back at £600 per month. Your dad is smart, there is an IHT exemption for any level of gift as long as it's regular income and as long as it doesn't affect your standard of living significantly. So, the repayments are tax free for him"
You have this the wrong way round. You are talking about the exemption for "normal expenditure out of income".
As you say, in the UK you you can make regular payments of gifts to another person and there’s no limit to how much you can give tax free, as long as you can afford the payments after meeting your usual living costs and also you pay from your regular income.
In this situation, it would be the OP that is making regular payments to her father. So it would be the OP's estate who would benefit from this if she were to die within seven years - not her father.
"So, the repayments are tax free for him"
They are tax free regardless (assuming there is no interest). If the payments were genuinely a gift to him from his daughter of £600 per month then they would be tax free for him regardless (well, unless the OP died within seven years and the OP had made very large gifts prior to this).
If they are repayments of a loan then return of capital is not taxable. You do not pay tax when you get your own money returned to you. But there would be tax to pay on any interest.
At most, I would guess this is a move to avoid any future care home fees.