Older stone walls, long pipe runs, no gas main and a survey team that has to drive there. None of that is a reason not to. All of it changes the design.
A heat pump in a rural Scottish house is a different job, and it should be quoted like one. The walls are often solid stone, the fuel being replaced is oil or LPG rather than gas, and the survey team has to drive to you. All of that changes the system design, the pipework, and the quote.