Originally Posted by
MPH1980
Not quite - the OP says they have an onward ticket on TK *from* IST.
Ah, you are correct - 2 letters makes all the difference ^
The advice above still stands, but as long as the OP tells the agent they are still travelling onwards to IST, they should not cancel the BA ticket. This is, of course, provided the OP has enough time between flights to manage this.