Need some advice on letting to buy

I am thinking of moving but I want to buy a house without the hassle of a chain and competition from other no chain buyers.

I have been thinking of getting an interest only buy-to-let mortgage with an LTV of 75% on my house which is fully paid off (no mortgage) and use the 75% equity and my cash as a deposit with a standard repayment mortage for my next house as I want to upsize. I would easily be able to pay the BTL mortgage and the standard repayment mortgage even if the BTL mortgaged house has not got tenants in it.

Now I know these kind of mortgages are known as let-to-buy but from what I can see, they are the same as BTL mortgages.

From what I know, the rent of the house would need to cover 125% of the monthly BTL mortgage payment. From my calculations, the rent would cover nearly triple the amount on interest only so not a problem.

As for getting another mortgage alongside the BTL mortgage, I understand that a bank won't mind and it should not affect my ability to get a standard residential repayment mortgage? The BTL mortgage can easily be covered by the rent.

I don't want to keep my current house though so the BTL mortgage would be temporary as I would want to sell my house as soon as I have moved into the new one. Would I have to put tenants in my house if I got a BTL/LTB mortgage or could I just move out and put the house up for sale?

Thanks.