Depends on the house. When I bought mine, I think the survey (which I have sadly lost) said something like 'the heating was not inspected but the house appeared to be at a comfortable temperature at the time of inspection; the boiler appears to be a number of years old and I would recommend you have the system inspected by a CORGI registered blablabla' and I bought the house, without further inspection, because the radiators got hot and I was buying toward the lower end of the market anyway.
If I sold my house today (still the same boiler), I doubt I'd get the cost of a new boiler back, so I'd probably sell it with the existing one in place. I may be wrong, though.