Apologies for the delay in response.
The screen grab is from a visit to the show home today. So it looks like it's Demon/Thus who are the provider for this line.
The PC was connected to a Draytek router, and into the phone port rather than anything wireless.
The linechecker on the demon website
https://demon.net/demon-business-lite-broadband/ suggests "you will get an estimated speed of 2.00 Mbps on on this telephone number." for the Annbank number provided by the office.
There is another number for the office which has a Prestwick prefix but the demon site suggests "Unfortunately this product is not available for 01292 844306 at the moment"
I don't know if the sales centre has a Prestwick number (01292 844306) which diverts calls to the Annbank number (01292 521613) or whether there are 2 lines in the office from different exchanges.
So, it looks like the router must be connected to the Prestwick exchange to achieve the speed rather than the Annbank one.
I haven't had any success through talking to anyone via the BT 'Chat' or calling a technical person within BT to understand if its possible to stipulate a connection to the Prestwick exchange rather than the Annbank exchange for a new line.
Screengrab as below