Originally Posted by
skyscanner
With separate tickets you are not transiting LHR. Just a ticket that ends at LHR, for which BA will be checking you are OK for entry to UK.
Then another ticket leaving LHR, for which BA will be checking you are OK for entry to Turkey.
If the second LHR-IST flight is cancelled or leaves without you (ORD-LHR is late) you will have problems
Note post 1 has been edited twice plus original