Most types of boiler are available as high efficiency or "condensing" boilers which are equipped with either a larger heat exchanger than normal, or even a second heat exchanger.
By recovering and using heat that would otherwise be lost up the flue, the best high-efficiency boilers can operate with seasonal efficiencies in excess of 90% compared to typically 78% for conventional types. Recovering the heat from the flue reduces the temperature of the flue gases to a point where water vapour produced during combustion is condensed out. A side-effect is that this condensate, which is usually acidic, has to be piped away to a drain or soakaway.
The Vaillant "ThermoCOMPACT" range of boilers (above) are designed to exacting quality standards - providing the best in reliability and performance.
The Grant "Vortex" condensing boiler - another popular choice.