It sounds like you have got a good deal. In similar situations some lenders will ask for a payment to bring the LTV ratio down to 75%, others will ask you to pay the settlement fee as well before putting you onto a mortgage deal that allows letting. Some buy to let mortgages use an interest rate 1% or so higher than residential mortgages.
It's up to you. You can ask them to be flexible but, in your shoes, I would pay them the £450 before they changed their minds.