Where Do I Go Through Customs or Immigration?

In London.

Immigration (passport check)will be in London, customs (stuff you are bringing into the country) will be in Newcastle. More than likely no customs officer will be on duty in Newcastle.

Passport Control (Immigration) and Customs in London. You will have to retrieve any checked bags you have in London and re-check them for the flight to Newcastle. London and Newcastle are both in England. When you return, you'll go through U.S. Customs and Immigration wherever you enter the U.S. And, I believe you are flying through Gatwick, not Heathrow--at least to the UK.

Ann, I'm assuming that you are flying into Gatwick (rather than Heathrow) since US Air flies CLT to LGW. If so, would suggest that you check the details under LGW's "Connecting at Gatwick" page. www.gatwickairport.com/check-in-security/connecting/ Regardless, the London airports have good signs directing you to immigration, baggage, customs, and connections.

"Passport Control (Immigration) and Customs in London. You will have to retrieve any checked bags you have in London and re-check them for the flight to Newcastle." If the connecting flight is part of the same alliance/airline, checked baggage will automatically be sent to the second flight. If this is the case then customs will be in Newcastle; even though London and Newcastle are both in England. A special tag will be placed on the baggage, and bags from international flights are segregated and sent to a separate conveyor in a secure part of the baggage hall. The passenger then collects the bags and passes through a customs checkpoint which may or may not be manned.

Trying not to confuse the OP...she is flying USAir to London and then, I believe, Flybe to Newcastle. The airlines are not aligned. Therefore, customs will be in London. Fortunately, both airlines are in the South terminal.

She said US Airways from London to Newcastle, so I made the assumption it was a code-share flight.

When you fly internationally, you'll always go through passport control at the first airport at which you land. You'll go through customs at whichever airport you retrieve your checked bags.