This is a private arrangement between the two of you and hence, it would not be regulated if it was a plain straightforward sale.
It amazes me that the people who regulate it are the FCA and they are the ones who would prosecute when you asked for advice, as to whether something is okay or not, they will say. Bizarre.
What you have here is a Private mortgage. The seller is in effect lending you the money and taking repayments over a period of time
What would normally happen is that the property would exchange and complete and you would grant the seller a mortgage over the property in favour of the seller and there would be a mortgage deed with the repayment terms. The seller would be acting just like a mortgage lender.
If the seller is remaining in occupation however this is a sale and rent back albeit that the rent is payable by way of mortgage and hence it is regulated.
Here is some reading from citizens advice https://www.citizensadvice.org.uk/debt-and-money/mortgage-problems/how-to-sort-out-your-mortgage-problems/sale-and-rent-back-schemes/
There is no reason why could not exchange contracts now and complete in some years time, but if I was acting for the seller, I would advise against that. It’s a very convoluted way of dealing with it.
Can I clarify anything for you?
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We can still exchange emails. Best wishes.