The UK has been a net importer of energy for over a decade, and as their generation capacity and reserves decrease the level of importing is reaching an all-time high. Their fuel price's dependence on international markets has a huge effect on the cost of electricity, especially if the exchange rate falls. Being energy dependent makes their electricity prices vulnerable to world events, as well.
Fixed-rate, long-term (contract) plans provide stability in electricity rates. If market energy costs suddenly trend upward where you live, you can rest assured that you won’t have to pay more out of pocket. However, if you want to switch to a different, lower-cost plan before the end of the contract term, you’ll likely have to pay a cancellation or early termination fee.