No, they are different: residential mortgages cannot be used to purchase Buy-to-Let properties, and vice versa.
There are specific Buy-to-Let mortgage products available from a number of lenders. Some will look at the borrower’s financial circumstances before agreeing to lend. Others will lend on the rental income alone, but they usually require that the rental income is at least 125% of the mortgage repayment, e.g. if the repayment is £800 then the minimum rental income would need to be £1,000 per month.