Tim Streater presented the following explanation :
Water doesn't always boil at 100C. Up a mountain where the air pressure is
lower, it boils below 100C. If the water in the CH pipes is at 1 Bar above
atmospheric (as it is here, f'rinstance),

So, 2 Bar, the pressure of a typical combi boiler. The pressure in a
combi boiler system is not there to permit hotter running, it is simply
to allow the system to be a sealed to atmosphere system. A combi
boilers flow temperature is still limited to 80C, more typically it
will run at 50C for best condensing efficiency.