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A bought a house with an unknown status boiler a decade ago. I could have asked for a boiler certificate but it wouldn't have made any difference to the house purchase as it would still have been 30+ years old and inefficient which was obvious from 5 yards away.
The boiler hasn't blown up yet. We looked into getting it replaced but the costs didn't justify the savings and apparently new boilers don't tend to last as long (lots of microcontrollers that fail and you can't get parts for after about 10 years compared to my hunk of iron with very little in the way of fancy controls).
If I were buying a house and the vendor refused a boiler survey I would pay for one myself. If the vendor refused access to do that I would get really suspicious.