At the moment the BoEBR is 4.5%, so you will pay 5% interest.
If the BoEBR falls to 4.0%, you pay 4.5%
If it rises to 5% you pay 5%.
How about a discounted tracker mortgage?
This is where for 2-5 years the interest rate will be a small fraction above the BoEBR.
After it will revert to a higher figure, usually the Standard Variable Rate (SVR, around 6.5% currently).
Note the caveats at the bottom of the page!