Ella Jessel, are you aware that the statement from the "Heatwaves: adapting to climate change" report, from which you have extracted the headline for this article, was made by the Mineral Products Association (MPA) who are actively lobbying the government against off-site construction because they are "largely made from lightweight systems and materials" i.e. not the ones they supply.
The principle of Modern Methods of Construction (MMC) as backed by the government is not materially or geometrically specific and the majority of offsite innovations are still largely fabricated from the same materials as traditional construction. Subsequently, it is a false logic and poor argument to suggest the 'hothouse' scenario is a direct consequence of MMC. It is the building regulations which need to adapt as this will inform engineered improvements to offsite products. Returning to antiquated construction methods to protect the profitability of members of the MPA is not progress.
Also, is the urban heat island effect not caused by the concentration of too many people and too much thermal mass? Cities don't have enough natural and lightweight/fast responding materials too allow them to cool down at night and surfaces with a high thermal mass continue to radiate heat?