Mark,I am of the same opinion/thought as you in that all the mugs that are unstamped/marked are NOT British issue but are either Nato or just commercial made copies.
At one time it was stated they were Danish but have had example of Danish mugs and these are stamped on the bottom.
Another question that has bugged the hell out of me is why are NONE(or none I have ever seen) of the bottles stamped/marked?
Again,like the mugs I have seen a Danish bottle,pretty much identical and that was stamped at the bottom.