To address urban growth, resource competition, and environmental degradation, effective integrated water planning is crucial. In the UK, policy frameworks like the 25-Year Plan for the environment and the National Framework for Water Resources stress the need for a systemic approach. Despite efforts in stakeholder engagement and meta-models, integrating physical and human aspects in water management remains a challenge. This paper introduces a multi-level framework for regional water planning, demonstrated through London's Sub-Regional Integrated Water Management Strategy (SIWMS). The framework, depicted as an inverted triangle, starts with conceptual analysis, gathering stakeholder insights and data. Integrated modelling creates a baseline for scenario assessment, providing evidence through metrics and simulations. Integrated planning focuses on collaboration for option selection and implementation. Results show the framework's effectiveness for systems-level analysis at the river basin scale. The logical progression facilitates stakeholder engagement, enhancing shared understanding. The Water Systems Integration Modelling (WSIMOD) Framework allows simultaneous assessment of interventions on various indicators, aiding in prioritising multi-benefit schemes and identifying potential negative impacts. The study supports the prioritisation of schemes like Sustainable Drainage System (SuDS) and emphasises the importance of a multi-level collaborative approach for robust, stakeholder-supported regional water planning to achieve effective implementation of planning and environmental policies.