I looked at heat pumps for my 5-bedroom detached house and the most expensive of the three quotes I had, including installation and any necessary changes to radiators, etc., was £18,000, or £13,000 with the grant. So I don't know where you got your numbers from. Granted, it's still a lot more than a boiler, but you were also comparing the cost of just the boiler with a full installation of a heat pump system.