Lets say I mail a letter from here in the U.S to my friend in England. Does a USPS plane fly it to the UK and unload it to a depot where it’s then handled by the British postal service? Or is there some type of depot here in the U.S where its loaded onto a British plane and flown to Britain?
However, if you’re asking “who owns the planes that international mail travels on”, that’s a different question. I believe that the space is purchased from other airlines in general, and that the USPS (or the Royal Mail) doesn’t own any planes themselves. I could be wrong, though.
The Royal Mail operates [one](http://www.airliners.net/photo/Royal-Mail-(Titan/Boeing-737-3Y0/1365813&tbl=photo_info&photo_nr=1&sok=WHERE__(airline_%3D_‘Royal_Mail_(Titan_Airways)’)_&sort=order_by_photo_id_DESC&prev_id=&next_id=1263526), although doesn’t own it, and it’s for operations within the UK.
The vast majority of international airmail just gets thrown onto commercial flights. The post office is just another customer - with the exceptions that it gets a good rate for sending so much stuff, and that it also will get some leeway in customs and security formalities. This latter is because we (I’m a postal worker) are trained to know what can go on a plane and what can’t, but the average Joe sending something unaccompanied isn’t; the benefit of this arrangement is that you the post office customer can send something overseas with little or no documentation (none in the case of a letter, or maybe just a tiny green customs declaration for a parcel). Essentially, the post office is trusted by the airlines to be the sender, and to assume the responsibility of the sender, relieving the posting public of that responsibility.
Another quirk of international mail is transit mail. If you have a letter from Australia to Mexico, for example, then being no direct flights, that mail will transit LAX, but it will do it in a mail bag labelled “Mexico”, and the USPS won’t open it. But in some cases, there isn’t even enough mail to warrant a bag, so a letter from Iceland to New Caledonia will just be thrown into the “Australia” bag, and we’ll open it here in Sydney, and the mail will actually clear Australian customs and all, be processed “land side”, and then be sent back into the international system for its final leg to New Caledonia.
I don’t know if this still happens, but British Airways planes (and before that BOAC) used to carry a “Royal Mail” logo on their fuselage to show they were the official carriers of mail leaving the UK.