I haven't read Alan Ross's Essay in Sociological Linguistics republished in Nancy Mitford's Noblesse Oblige since I was in my first year at university. I am very ashamed to say that avoiding Non-U (i.e. non-upper class) words and phrases is the most lasting effect of my university education. However I am delighted to discover it is U (upper class) to say England to mean the UK and Non-U to say Britain.
Saying England instead of Britain is that's going out (though Romanians keep up the tradition and talk about the English Ambassador). It's a good thing to do for several reasons - for example it annoys false pedants.
I mean absolutely no disrespect to Scotland, a great country that I love with all my heart and hope one day to visit. But England has been used far more often than Great Britain (or worse Britain) to mean Great Britain or the UK since 1707 when the two countries merged and therefore England is slightly more correct than Britain. Disraeli signed the Treaty of Berlin as 'Prime Minister of England' and Churchill always spoke of England not of Britain. So did most people, apart from the Scots, the Welsh, Northern Irishmen and Americans, until fifty years ago.