Asbestos in benign form is harmless, it's only when the fibres are released because of a breakage of the material that they form part of, that they can become potentially harmful.
In enclosed spaces, the fibres can hang in the air, and if ingested have the potential to be highly carcenogenic. However in the open air they would blow away and the chances of ingesting a fibre would be highly reduced.
I would guess that your potential house is a pre war build? If so then they were built to last . Roof tiles break, but I can't see any reason why you could not have a broken tile replaced under an insurance claim dependent upon the circumstances of the claim. I guess the onus though would be on you to disclose to your roofer that your existing tiles have an asbestos content. Asbestos roof tiles are not uncommon. However if you were considering extending your property as some piont in the future you may well want to factor in this finding.