40 years ago there were no domestic condensing boilers, they were standard efficiency cast iron or lead washed copper exchangers with burner beneath the heat exchanger. The reason flow temps were high was to avoid condensation forming on the cast iron if it ran for a time below dew point (50 degrees-ish) and rotting it out or dripping onto a burner. Flow 75, return 65 = no condensation.
A condensing boiler is always more efficient than a non-condensing boiler by a margin, a condensing boiler condensing on the exchanger is more efficient than one releasing its latent heat at the flue terminal but the margin is far, far smaller as are the monetary savings.
The majority of systems are installed with the standard rad flow and return temps as per catalogue so boilers will be run at those temps to achieve catalogue outputs and most systems were installed years ago in to run hot therefore that's why manufacturers make them still. The percentage of U/F systems is miniscule in comparison with radiators.
The benefit of modulating controls are when high temps are required the boiler runs hot, when lower outputs are required the flow temp and the return temp respectively are reduced leading to condensing within the appliance; the rads stay the same size though the customer will sometimes grumble when the rads are cooler rather than baking hot.
10% improvement in high temp operation will be 10% of over 10.7KW of sensible heat from a cubic metre methane; 10% of latent heat recoverable when flue products condense within the boiler represents 0.1kw of the around 0.9kw from the same amount of fuel. Look at the cost of a cubic metre of methane. Look at the cost of an underfloor installation to a radiator system (I'm told and happily believe the comfort factor is higher with U/F; controllability, I'm less convinced).
None of this will save the planet but everything combined does help a bit.
Manufacturers efficiency figures are derived on special rigs with test gas at higher pressures and a standard operating format different to although supposedly representative of the way we run heating systems.
