What do you mean by "5% mortgage protection interest rate" and are you talking about mortgage protection life insurance or mortgage repayment protection insurance?
While 6% tends to be the standard, I would doubt if too many bank officials would notice the difference between a 5% policy and a 6% policy if it's presented to them as part of the security on a loan.
Clubman - the rate of decrease of cover on a typical Mortgage Protection Life Insurance policy is based on the rate of decrease of capital on an annuity mortgage at a constant interest rate of 6%. If the actual rate of interest on the mortgage is less than 6%, then the mortgage capital will be decreasing at a slightly faster rate than the cover on the policy. So the capital on the mortgage will be lower than the level of cover on the policy, which is as it should be. But if the rate of interest on the mortgage is greater than the assumed rate of interest on the policy, the cover may be going down faster than the mortgage capital, leading to a potential (small) shortfall in the event of death.