$100k == £60k. 60 less 33% / 12 = 3300 take home per month. London rent for 2 bedrooms is about £1500-2000 anywhere you would want your kids to go to school. £100 for commuting. £200-£300 for food. Car £150. Local tax £200 ...
All that said, the point is that £60k/$100k isn't a common salary for this sort of full-time worker outside of management or in an early stage of their career (<10 years). Clearly people are managing on a lot less because that's what people are earning. The most common solution for the older family types is to have multiple earners in the home which, again, splits the costs down. I can't think of any British family I know around my age where both partners aren't working except for me and my wife..