This is a variable mortgage product, either a tracker or discount rate, which has a maximum ceiling interest rate. The maximum rate chargeable will be shown on your mortgage offer. These are not very common in the current market. You are, however, likely to see what are known as collared rate mortgages, particularly if you have a tracker type product.
Your documents will state that the interest rate you pay will never be lower than a specific minimum rate, no matter how low the BBR goes.