Having handled gas for 20 years I make a variety of adaptors and will fill any bottle. Most dive shops will fill a cylinder for you if you turn up with the correct adaptors for the cylinder. It is simple to make a whip for a CGA450 to SCUBA DIN477 fitting as it is for a bull nose to CGA 540 (O2 J cylinder). It is also simple to make an adaptor for converting a CGA540 to DIN 477 so the SCUBA shop can attach the filling whip direct to the cylinder.
The main problem occurs because the US cylinders are DOT standard cylinders and are not CE approved. This causes a problem for any gas filling station filling a non CE marked cylinder (despite them being the same cylinder with different paperwork). Another example of the problems caused by trying to bypass Europe....
However if you come prepared with the right adaptors and tell them it is an O2 kit you will get it filled. I will fill any cylinder provided it is in test to whatever the max working pressure is required (I have a gas booster). An average aviation O2 cylinder is about a fiver to fill.
All of the Mikes Waterfront Warehouses around the UK will fill an O2 kit as well if you have the adaptors. If anyone has a problem PM me and I will make a call.