The first thing is that the increased mortgage on your original house will have to be taken out on BTL terms - that means because you are planning to let the house you have to take out a new BTL mortgage. The terms of these are less favourable than normal mortgages and command a higher interest rate.
The best BTL deals you can obtain will require you to have a deposit of a minimum of 20% and for reasonable rates you will be looking at 25-30%
So this means that the most equity you will be able to release is 75% in the original house.
You will probably only be able to get a repayment mortgage because interest only mortgages on BTL do not exist
Say your house is worth £1m. You will be able to get a BTL mortgage for £750k on 20 year terms. At an optimistic 5% interest rate your repayments will be £5k a month. Say your rental income is £8k a month. The deductible interest component of your mortgage repayments could be say £1k a month.
Except when the tapering laws have gone actually none of your mortgage repayments will be tax deductible. You will be paying tax on your rental income at 45% assuming the rental income chucks you into the highest rate tax bracket - so let's call that £5k a month.
So what would happen is that you would have mortgage repayments of £5k a month and a tax bill of £5k a month and rental income of $8k a month.
This is why most BTL landlords are now either paying off their BTL mortgages or consolidating into property companies.