As for the 'integrated urban drainage model of London', we should be looking to establish different stocks and areas to help us deal with water (quantity and quality) issues in an integrated way. A drainage model and a surface water flooding model would be at the heart of a System Dynamics Model (SDM) approach - see this introductory paper outlining methodology.
The entire area needs to be divided into drainage zones and infrastructure (new and proposed) interconnections between them, to be able to replicate (globally) the behaviour of the systems. The 'stocks' in each of the drainage areas could be pervious and impervious areas (these could be divided into those suitable for SUDS retrofitting, or even could be considered for retrofitting, but with different associated costs as some would be more/less difficult to retrofit - this requires a discussion and lots of thought). Each of the SUDS option would have costs associated with them (both CAPEX and OPEX) as well as associated reduction in runoff (and/or improvement in water quality). Again, all these will need appropriate rates (again needs careful consideration and discussion with stakeholders).
After creating such a model and running to check it produces expected (explainable) answers, the application of different interventions needs looking into, e.g., SUDS, SUDS + smaller/shorter tunnel(s), or the big tunnel, etc., to see what long term consequences can be envisaged.
In addition to technical and economic modelling and options, socio/political options need to be considered, e.g., willingness of Londoners to embrace new technologies, along with policy levers (subsidies, payments for installing SUDS in existing buildings, etc) and how those could influence the output of the model.
The model can/should be built incrementally with involvement of all stakeholders (Thames Water, Env. Agency, technical experts, citizen action groups, etc).
Complex models, such as SDM, are possible and provide great insight but great care and consideration is required in their construction, solution and interpretation, all of which is possible for a city as large and complex as London.