The British power market is most heavilly influenced by the wholesale gas market and the carbon price. Like power, gas is traded short term on a balancing market operated by National Grid. Due to "linepack", the relatively flexible volume of gas that sits at one point in time within the pipelines, balancing supply and demand is much easier than electricity.
The most important benchmarks for future power prices are the latest future contract prices for natural gas. In Britain the standard contract is NBP, this is equivalent to the Dutch TTF. Awkwardly, the NBP is still priced in pence per therm, instead of the more convenient £/MWh that electricity markets are priced in. We've done these conversions for you!