Not sure about standard sizing in the UK, however, in the US in general if someone is talking about a 16x20 frame it usually refers to the image size the frame will accommodate. However, 16x20 will not be the visible size or the frame size or even the mat opening size. If there is no mat then 16x20 size means that the glass and backing board would be 16x20. The inside of the frame is typically cut to 1/16 to 1/8 of an inch larger so there is no binding. The outside of the frame size can be anything from 1/8" to over 5 inches larger based on the width of the picture frame moulding. Also, the visible size will vary from 1/8 of an inch to 3/4 of an inch smaller based on how much the frame overlaps the glass. Visible size cant be the same size as image and glass size since the glass would fall through.
If you are using a mat and the mat opening is for a 16x20 print the opening will usually be 1/2 inch smaller although my company makes most 1/4 inch small so more of the image is visible. Of course in the case final frame size is based on the mat borders and frame width.
Sound confusing? It is, and as the owner of a frame supply company I wish it were less so.