Heat Pumps (HPs) are recognised as a key technology to decarbonise heat in buildings to reach the UK’s Net Zero target by 2050. A 100% uptake of HPs into existing building stock and operating in cold winter weather similar to 2010 would result in a peak demand of 132 GW which is more than double of existing demand. Such an increase in peak demand presents significant challenges for the electricity system and network operators at both local and national levels. The challenges include overloading of the existing transmission & distribution (T&D) network from peak demand, excessive voltage drops, and inadequate thermal capacity at substations, which can lead to overheating and reduce system reliability. To overcome these challenges network operators would need to make significant capital investments for network reinforcement and increase balancing services. Such capital-intensive electrification programmes could not only hinder decarbonisation activities but also increase fuel poverty for end users. Domestic demand-side flexibility has been identified as a valuable addition to the energy system to defer infrastructure investment required to meet increasing peak electricity demand, match the output of variable renewable generation, and a source of income to offset high electricity bills. The overarching aim of this research is to develop tools to forecast and quantify aggregated HP demand and flexibility. Moreover, we strategically allocate the heat pump demand flexibility to provide multiple electricity grid services. This includes understanding the technical and commercial challenges of participating in multiple grid services as well as various interactions between grid services at the T&D level. This project will specifically focus on two study areas of Manchester and Belfast. This project is novel in its comprehensive approach to quantifying the flexibility available from heat pumps at both the substation and national levels, utilising advanced AI and physical models. The project uniquely optimises the capabilities of heat pumps to provide multiple flexibility services, considering both T&D networks. The expected benefits include decarbonisation of heat, improved electricity system reliability, business opportunities for flexibility providers, and cost savings for end users and network operators.