Natural gas has become the UK’s foremost primary energy source, providing some 39% of our energy needs. The National Transmission System (NTS) has developed from its humble beginnings when natural gas was first discovered in the North Sea in the 1960s to become a complex interconnected network delivering up to 550 million cubic meters of gas daily. Gas has also become an increasingly important energy source for power generation, currently generating 35% of our electricity. This presents major challenges for the planning and operation of both the electricity and gas networks as their interdependence grows into the future. With the government’s goal of drastically reducing emissions from power generation by 2020, Combined Cycle Gas Turbine units, and therefore the NTS, will have to offer a new degree of flexibility to quickly respond to the intermittency of the growing penetration of wind generation on the electricity transmission system. Coupling this with the decline in the UK natural gas resources resulting in the NTS becoming reliant on imports to meet demand, it is becoming increasingly difficult to decouple the security of the gas supply from the security of the electricity supply in the UK. This study presents the modelling challenge of assessing this growing interaction and provides a robust methodology for completing a security analysis using detailed network models of the UK gas and electricity transmission systems. A thorough investigation of the intraday operation of the two systems in 2020 is presented given the growth of wind generation in the UK. The results are analysed and the implications for combined modelling and assessment are discussed as we enter a new era for UK energy security.