I have successfully booked IST-LHR, as a TK flight number, just that one segment, on united.com. It took a while to issue the ticket; I got a "partner ticketing delay notice" e-mail and then about 24 hours later I got a ticket. Despite not having any UA segments it showed up just fine in my "my itineraries" list.

Just guessing here, could it be that when UA tried to ticket your flight, TK denied that the space was still available, and so they cancelled the reservation? Buying space on another carrier, there's a built in 24-48 hour delay where the carrier has to confirm the inventory.

UA should be able to sell you a flight entirely operated by other carriers, but it is *not* something they're accustomed to doing, so no surprise the agent told you something that sounded plausible.

(The ability of united.com to sell you TK flights is really nice because TK otherwise has a ~$20 booking fee that they apply to flights they sell on their own Web site and most online travel agencies bundle that fee, too -- united.com offers the best deal by far. It would be too bad if UA stopped doing that!)

I think your terminology in the title is a bit confusing here, since you didn't exactly buy a reservation with all codeshare flights - you bought a reservation with only partner carrier flight numbers (no "UA" flights) and no codeshare flights at all (no "UA flight number on a flight operated by Lufthansa or Turkish Airilnes", etc.).