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Are you not using PHP? If you needed to add or remove fields after the page loaded, that's one thing, but if you know ahead of time then you should do all the work in PHP and deliver the completed result to the browser without it having to perform additional work.

If ($lat, $lng) is the centre of the postcode area, then this will find the locations of properties within $d Km select lat, lng from properties where pow(lat-$lat, 2) + pow((lng-$lng)*cos(radians($lat)), 2) < pow($d/110.25, 2)

They have a big database of locations. Probably purchased from and/or subscribed to a service that tracks stuff like that so they don't have to try to stay up to date themselves. They know the latitude and longitude of an address, also probably from a third-party service (eg, Google Maps). A postcode has a location too, though it's obviously not quite as precise. Then they can do some pretty simple math to estimate the distance between the two points.