The effects of increasing water and energy demand pose a growing threat to national infrastructure strategies. Within the UK there is concern that a future lack of water will compromise the UK's current energy policy to meet an increasing demand for electricity by more thermal generation. This paper investigates this by modelling the water demand of the UK's thermal generation in 2030 and 2050 for several future electricity generation pathways. Unlike previous studies this paper has obtained water abstraction and consumption figures specific to the UK. While the water demands were heavily pathway dependent this study finds for the thermal generation pathways there is a serious mismatch between the assumed freshwater available at 2030 and 2050, its expected actual availability, and an understanding of the implications this has for future generation costs. It is shown that a solution is to make greater use of the UK's seawater resource. This study finds the emphasis UK energy policy gives to the competing poles of low cost electricity generation and environmental protection will have significant impacts on the cost and make-up of the UK's future electricity generation portfolio. A companion paper will consider the generation cost issues if seawater is not available.