The French phrase "par avion" literally translates to "by plane." What it means is that something is being air-mailed to another country via plane. Almost anything and everything you ship to another country over a large body of water from the United States is probably going to go "par avion." This is the stamp of approval for all shipping to the U.K. from the U.S., and it goes something like this.
All Envelopes Are Stamped and Prepaid
If you are sending letters and insulated small envelope packages, you address them as you would any other piece of mail. Then you take it to your local post office, point out that this is going to the United Kingdom of Britain, and the postal worker weighs your envelope. When he/she types in the address, the scale and the computer will tell you how much postage is due to air-mail this letter or small package envelope. You pay the postage, the postage is printed and stuck to the envelope (do not be surprised at the cost; it is not cheap to air-mail anything), and it is placed in the back for sorting.
Larger Packages Have to Be Wrapped in Boxes That Are Not Confusing
It is a strange sort of outdated type of packaging, but you still have to make sure every box is plain and does not have any other bar codes or postal marks on it if it is going to the U.K. It has something to do with how they sort things over there. When your package to the U.K. is clearly a box or rectangle sort of package, and it weighs less than a certain amount of pounds, you have to make sure that there is nothing else on it that can confuse the system or delay the delivery. Then address it, weigh it, and stamp it.
Anything Too Heavy Cannot Be Air-Mailed
There are weight restrictions on packages going by air mail. There are also content restrictions, so make sure you follow those guidelines or the packages will never reach their destinations. If the packages are particularly heavy, they cannot be sent "par avion," but rather they go by ocean freighter. If you are sending something really, really heavy (e.g., a quarter- or half-ton or heavier!), then it definitely is not flown over the ocean to Britain. It has to go by freighter, and you will have to make very different arrangements to ship the item over to Britain.
Learn more about shipping to the UK from the US from shipping centers.