If a loan is taken out against a property there is a legal charge placed onto the property. The charge is the pecking order in which lenders get paid if the house is repossessed and sold. As you have no mortgage on the property any charge that is put onto it will be a First Charge, if you went on a did further borrowing this would then go as a second charge etc.
If you have informed the lender that you are going to let the property out then he will have arranged (or should have) a Buy to Let mortgage for you. A secured loan is often something that is placed onto a property as a second charge on a property (payable after the mortgage which would always be first charge)
You also have to understand the difference between the 2 companies also. Nat West are a High Street lender who deal with customers who have good credit. This is the area of business that all High Street banks offer in and it is very rare that they will touch anyone who had even slightly adverse credit.
You have come out of an IVA which is just about as serious a credit problem as you can get for an individual, in my opinion an IVA can be more damaging than a bankruptcy for the time it will take to start gaining meaningful credit terms again. This is the position you are in. It is quite possible that for the simple fact that you have held an IVA in the past that Nat West may never allow you to borrow money from them again. While after 6 years the IVA will drop of your credit file most lenders will ask if you have ever been made bankrupt or held an IVA and you are legally obliged to declare it and to not do so can be classed as fruadulent. There are several High Street banks that have this policy. For the next few years you will find that the only lenders that will entertain you will be lenders that charge high fees and high interest rates for lending money to you. This will slowly improve though
Before accepting the First Charge loan I would still go and see a mortgage broker to see if you have access to any other lenders as it would be most likely that if you are accepted by anyone else it will be at a lower cost on fees and interest rates