Tom Scott talks about it as though "combi boiler" are completely ubiquitous on newer housing in the UK today

Tanks were rare for about 40 years, as instant "combi" burners were popular on cheap housing and most UK housing is cheap junk. The big housebuilding companies didn't design heating systems well and if they messed up, they'd have a more powerful burner than needed for heating anyway, in order to do instant water heating and avoid taking up space with a hot water tank or caring about pipe run design much. The number of small homes with 30kW heaters is scandalous, but why would the builder care if the heating is even more inefficient when gas is cheap and the resident pays the bills, not the builder?

That's starting to reverse because it's easier to get permission for homes with heat pumps and insulated water storage now, due to tighter pollution rules and so on, but the big developers still don't have great experts and there's 40 years of junk out there now.