Personally I think it is due to constant and unrelenting attack on the concept of being British.
First there was the IRA saying "Brits out".
Then surprisingly you have Margaret Thatcher. I can remember when there was British Rail, British Steel, British Telecom, British Airways and so on. None of these made a distinction between Scotland, England or Wales. Then privatIsation occurred and even if some of these still exist they often use the letter B rather than the word British.
After the British Empire disappeared a lot of people started saying it had not been a good thing. They were probably saying earlier but nobody was listening.
Even the British Commonwealth is politely referred to simply as the Commonwealth.
Now that Scotland and Wales have a parliament (and sometimes Northern Ireland too!) a correction is often required when talking about the British parliament.
So now you have lots of people who thought British and English were more or less the same thing being constantly corrected (and more so than before).
"I think you mean English dear".
Just my thoughts on the matter.