Combi boiler overflow pipe outside constantly leaking,why? Boiler pressure is ok

It's not an overflow pipe, it's a condensate drain, it's meant to drain water.

It MIGHT NOT be a condensate drain as these should be run to a drain or soak-away and not just let run outside as the water from condensate water from is slightly acidic if its not a condensing boiler and its dripping water from the pressure release vent this could mean that the hot water heat exchanger unit is leaking between the primary and secondary coils thus increasing the pressure and causing it to vent outside through the release valve the only way to be sure is GET IT CHECKED.

is it a plastic pipe, then it's probably a condesate pipe which should be running to a drain, and they do drain condensate from the boiler whenever it is in use, the pipe should not be allowed to just drain onto the floor etc