"I would have just put a condensate pump on the drain line and routed it out a window to drip continuously somewhere appropriate."
UK building regulations for domestic properties insist that the overflow from a cold water cistern in the loft comes out somewhere you can't ignore it, like above the front door.
When the builders have skimped on lagging the cistern, said cistern freezes in winter, rendering the float valve inoperable. If you are unlucky, the water overflows through the ceiling. If you are lucky, the overflow pipe does its job.
Ooh look, sheet ice just outside your front door.