Interest on main residence and buy to let flat.

Five years ago I was made redundant and invested the cash and part pension lump sum into buying a flat to rent out. I've paid no interest so haven't claimed any tax relief. I have since bought two other flats with mortgages that I let out and claim relief on the interest for them. The mortgage on my main residence is nearing the end of a fixed term and I need to remortgage. I pay interest only on this (my main residence) with the flat I own outright providing the guarantee. The value of the flat I own and the outstanding mortgage on my residence is much is similar so I thought about taking out a buy to let mortgage on the flat to pay of my residential mortgage and then claim tax relief on the mortgage payments on the flat. Is this something I can legally do? The reality is that the mortgages I have been offered on the rental flat are poor value due to the 80% LTV of a buy to let and remortgaging my house at a 20% LTV is much cheaper. Can I effectively remortgage my house and lend this to my business at the lower interest I receive from my residetial mortgage? Hope this makes sense!

Hi
Thank you for your question.
You can only claim tax relief on mortgage interest paymens on properties that you currently let out and are subject to a buy to let mortgage.