Even though I just booked my last ticket right before the office closed (still to be picked up, though) my advice is:

1. Book your ITN with BA RTW (or some other OW facility)
2. Get it validated and sent to the price desk for ex-Turkey price
3. Contact Bremen by phone/fax and ask them to do the ticketing, send/fax them your CC info etc.
4. Ask them to send the ticket to BA IST airport counter/office for pickup (if it's a paper ticket, with e-ticket should be even more straightforward)

It's always good to keep booking and ticketing as separate issues when dealing with small/remote OW locations.

This should work out just fine.