You mean in the sense that "BRITT: OMN: REGINA" was still on the 1953 coinage specifically, but the "BRITT OMN" disappeared from 1954 onwards? I assume they dropped it because it was the wrong meaning - although I don't know for absolute certain. Could certainly have been replaced by just "BRITT REGINA" or a contracted version of that. As per Victoria with "BRITT REG" - "Queen of all the Britons".
As far as no national markings like "GREAT BRITAIN", we never really have had. "BRITT" was probably the furthest it got. I suppose the reason is historic, but could be interpreted as either self important or self effacing, depending on how you view it.