The decarbonisation of the UK economy, driven by the national net-zero target, will require the large-scale electrification of important fields within the energy sector, such as transport and heating. The increase in electricity demand associated with this process will mean that flexible and integrated energy systems will be increasingly relevant in the future, especially in terms of improving the efficiency of energy use and alleviating stress on the electricity grid. District heating is a key technology in this process, particularly in densely populated urban centres, as it enables the integration of heating with other energy vectors and unlocks the potential for urban waste heat recovery to deliver significant carbon and cost savings. This paper aims to investigate how the benefits of a waste heat recovery system can be maximised through its flexible operation. This will be achieved by modelling the coupling of thermal energy storage (TES) with a district-scale heat pump, based upon time-of-use electricity prices and the concept of an existing system that recovers waste heat from the London Underground mass rapid transit system. The results from this investigation are reported together with recommendations for further development and future deployment of heat recovery from subway systems in general.