They have secured a large sum of money over your property and don't want you to do anything which could adversely affect their security.
When they lend on a buy to let mortgage they usually check the rental valuations etc... to make sure there is a reasonable chance of the mortgage being covered by the rent and less chance of arrears.
Most lenders will allow rentals but usually you would need their consent first and have to make sure that any tenants are on a formal tenancy.
If the lender repossessed they would want everything on an official footing so they can sell the property on easily.
People in the property who are not on a formal basis can be a lot harder to shift. At least on a formal tenancy they can give peoper notice etc... in accordance with the tenancy and have something to act on.
thanks for the help, this answer bank thing is brilliant! i was thinking about going travelling for six months and renting to a friend. if she couldnt pay the rent which would cover the mortgage i do have the money to cover it each month just in case. would ost lenders be happy to swap your mortage over to a buy to let? and then does that cost loads?
I can let out the property i own without changing my mortgage, the lender said there was no risk and was happy to let me rent it out as long as they knew. Other mortgage companies will change the interest rate and charge set up fees etc.