If your variable rate deal has a mortgage collar, there is a limit to how low your variable rate can go. This may be the initial rate you are offered. So if the Bank of England base rate falls below the minimum interest rate set by your lender, you will pay a higher rate of interest than that.
At the other end of the sartorial scale, a mortgage cap is the maximum percentage you would pay, even if interest rates exceed that. Always check if these rate limits apply to variable rate deals you are considering.