The paper explores the impact of urban geometry on solar availability in twenty-four urban forms of London. The morphological analysis of the urban forms was based on their digital elevation models (DEMs), for which density and six urban form descriptors were computed using image processing techniques. Solar radiation and mean radiant temperature (Tmrt) simulations were performed using PPF and SOLWEIG softwares, respectively. The statistical elaboration of the results reveals a strong correlation (r>-0.950) between density and mean ground and façades sky view factor (SVF) values. Furthermore, among the urban form descriptors considered, mean outdoor distance, site coverage and frontal area density were the most influential for the solar performance of open spaces; whilst solar availability on building façades was mostly affected by frontal area density and standard deviation of building height. The influence of the orientation of urban forms was found to vary with the sun’s altitude: the lower the altitude, the greater the influence. The seasonal performance of the urban forms in enhancing outdoor thermal comfort was evaluated through solar availability and Tmrt in open spaces. According to the climatic data of London, for 87.5% of daytime hours solar radiation would enhance outdoor thermal comfort. In this context, urban forms of lower density perform in general better allowing more solar radiation on the ground. However, as increased density is an objective for urban environmental sustainability, design solutions for enhancing solar availability in high/medium densities were explored. According to the findings, some general guidelines can be identified: i) lower coverage (and thus taller buildings) is preferable, ii) building forms should be designed with minimum undulations and iii) vertical and horizontal randomness is beneficial.