The Balsa Cabin list 2mm. birch plywood on http://balsacabin.webs.com/ They are in Malden, Essex and are a 100% reliable mail order company. It might be a problem though posting it to you in the size you want. I don’t know whether it could be classed as aviation quality. Most ply used by modellers is flexible with one of the grains at 90 degrees to the other two. I believe aviation quality ply is referred to as square with the three grains at 45 degrees to each other.

The Balsa Cabin is excellent. I get all my wood from there. In the past if I have needed larger sheets I called them to “order” the uncut sheets, or cut them to the size I wanted. They were very helpful.

The quality of their wood is excellent too. Don’t go for the “lite ply” as this is not suitable, but all of the other ply is really good. It planes/sands/routs to a very high accuracy.