ok so, per https://www.gov.uk/government/statistics/english-housing-survey-2022-to-2023-energy/english-housing-survey-2022-to-2023-energy-report the modeled heat requirements are 231 kWh/m^2/yr, so that means your cold-start train could service ...a little under 26 square-meter-years' worth of heat.
So, a hundred tube trains would give you 2600 square meter years' of domestic heat, under UK gov't models.