BA website would only sell from Turkish POS in this scenario though you are right that Expedia UK would work in this case

I actually don't think TA can "shop" POS either

If your corp TA / procurement tool is like ours they're also making sure you adhere to travel policy, providing approval workflow, managing reporting, linking up with International SOS to manage where we all are at any one time etc. We also get free seat selection on BA flights and we get a volume based rebate from our major preferred carriers. So although sometimes you could find a better deal yourself in the overall wider scheme of things it works out better for your organisation.

As an example we did drift away somewhat from booking hotels through the portal. Turned out people were booking their preferred chains, more expensive packaged options etc. Unfortunately whilst the good folk of FT are trustworthy a lot of others aren't! Now it's been tightened up so if you book air without hotel it'll generate an exception