We represent hourly, regional an wind data and transmission constraints in an investment planning model calibrated to the UK and test sensitivities of least cost expansions to fuel and technology prices. Thus we can calculate the value of transmission expansions to the system. We represent limited public acceptance of wind and regional network constraints by maximum built rates per region and year. Thus we calculate the marginal value of improved planning and grid connection regimes. It is likely that some constraints will remain. Market designs that do not allow for regional differentiation to reflect transmission and planning constraints can increase overall costs to consumers.