Hi, I'm planning to send some prints to friends in the UK, and I'm considering sending them matted (but unframed because of the shipping cost and risk of glass breakage). My first instinct is to mat them in 11x14 or 16x20 (outside dimensions) mats to fit standard sized frames. But then I realized that standard size here in the US isn't necessarily standard size in the UK. Or is it? So, should I mat for standard "imperial" sizes, "A" sizes, or metric sizes, for maximum convenience for the recipients to go out and find a frame?