It's not likely that interest rates will jump in the short term though Brexit may make a liar out of me. If you fix you are fixing for the long term possibility /probability that interest rates will increase. Its purely crystal ball gazing & anyone talking about interest rates in 4 to 5 years are purely guessing.
I've never fixed in my life but I still love the idea of fixed rates. It brings certainy and reliability to your life
Variable rates are more tricky as they are not linked to any index or external authority. Banks can set them as high as they wish so it's more difficult to predict what they will be in the near future.
In general they may increase because of two reasons. First is the same as with trackers, ECB increases the rates, local banks likely follow. Second reason would be when local banks get into financial trouble and need money. They can increase rates regardless of the ECB rates.