According to Samknows, KA6 5HL is served by the Prestwick exchange. The checkers indicate fibre should be available by December and the estimated speed is 24.6Mb down, 5.7Mb up - which chimes pretty closely with what you got.
It is right on the edge of the exchange area though:
On the other hand the phone number itself as you say, seems to be served from Annbank so while a bit puzzling I suspect it's a BT fibre connection. It's remotely possible, though unlikely they have more than one line at the premises and which are connected to different exchanges - or that the number they gave you wasn't the one they're using for broadband.
In any case if they were indeed using ADSL from the Annbank exchange that'd be needing something like 10 bonded ADSL lines which just doesn't happen. Whereas those speeds seem spot on for a fibre connection from Prestwick, which the postcode checker says is what covers 100% of that postcode, and seems a far more likely scenario.
Or it could just be the number was transferred or reallocated in the past and BT's records haven't been updated properly to reflect the exchange it is now connected to.
---------- Post added at 23:50 ---------- Previous post was at 23:40 ----------
Assuming (as you're talking about "plots" here) that the prospective home hasn't actually been built yet, I would suggest getting in touch with BT with regard to the phone line installation - you may be able to negotiate exactly which cabinet the line comes from and dictate fibre as a requirement, as it's a new build and they will be installing a new line from scratch anyway. That's the only way to be sure I'm afraid.
That said, it seems almost certain at least part of the development will have fibre coverage so at worst you could buddy up with a nearby neighbour who can get it and relay it to your home wirelessly
