Normally in the U.K. they are fixed for a period of time - 5 years is quite common, but you can get 10 and even full term ones. You can also get ones which track the base rate.

Typically people refix after the term. My 5 year recently ended and my monthly cost increased as I’m now fixed at a higher rate for another 5 years - at 4.5%, although it’s still less %age of my wage than it was 5 years ago.

During the fixed term you can’t repay it without a penalty, but if you move house you can port the mortgage so it’s secured against the new house instead.