An HMO mortgage is a type of mortgage specifically designed for HMO or multi-let properties. It allows for the letting of the property under multiple tenancies under the terms and conditions of the HMO mortgage.
Normal BTL mortgages do not allow for the letting of property under multiple tenancies under their mortgage terms and conditions. Subsequent to this, the letting of any property as a multi-let or HMO, but using a normal BTL mortgage will likely breach the terms and conditions of a normal BTL lender.
If a BTL mortgage lender finds out that this has occurred (via the HMO register for example) the consequences can be severe. The repercussions can include but are not limited to the lender having the ability and legal right to recall the mortgage with little or no notice. Your credit rating may also be at risk, and you will unlikely be able to gain any lending with that particular mortgage lender.