First of all, although you may not have been aware of it it does sound like you have been commiting fraud for a little while. It is actually illegal to hold a house on a residential mortgage while you are renting the house out. Lenders will often give you specific permission to do this (on request only) in cases like your own where you have bought a second property but this permission will normally end when you hit the SVR forcing you to take on a Buy to Let mortgage.
It is a Buy to Let (BTL) mortgage that you need to be looking at. The good news about this is that the calculation for affordability is not based on your earned income but is based on the assessed rental of your property. Saying that, you will need a minimum of £20k - £25k earned income between you to qualify for acceptance for a buy to let application.
The way the affordability is calculated for buy to lets is that you will pick your product that you want (speak to a mortgage broker) and then this rate is calculated to give you a monthly cost. The rental you receive (or assessed to be able to receive) must then be 125% of the mortgage payment. So if your mortgage payment was £400 per month then the lender would want you to be assessed for £500 rental income per month. This calculation is based on the Interest Only price of the mortgage, not a repayment mortgage.
Fees on BTL products are quite high at present but the rates are very keen and in a lot of cases (with low loan to value) just as competitive as residential mortgages
Good Luck, hope this helps