I don't see how installing underfloor heating in an existing house can make economic sense. Additionally in older houses the windows tend to be a bit leaky, so

I don't see how installing underfloor heating in an existing house can make economic sense. Additionally in older houses the windows tend to be a bit leaky, so the radiators being under the windows is important to prevent unpleasant cold drafts. Yes, the heat pump will take an efficiency hit for using radiators rather than underfloor heating. I'd say that's just part of the cost of living in an old house.