I wasn't aware that any preference was given to "locals". You ought to see how it works... Near the Heathrow suite at TC there is the Traffic Manager position. There's a radar display there, set to about 100 miles range, showing only Heathrow inbounds. The TM simply counts the a/c heading for, say LAM, and decides that x is enough... so the next one gets stack swapped. There ain't nothing super-scientific in it and I don't know anyone who is biased towards "locals". If BNN fills up we get a lot of US airlines moving down to OCK.