Update on ordering on the US Mr Porter site from outside the US:
My 2 Raf shirts arrived today. One of them fits perfectly, very happy. The other one I guess I could wear as well but ideally would be 1 size smaller.
1) Returning the shirt. Unfortunately, it turns out that:
"Outside the US and Canada, customs duties and sales taxes are non-refundable through MR PORTER."
I guess I could have looked that up before I ordered (2 mouse clicks, really), but when...