Britain - King Edward VIII Late Usage Cover

I'm always saying 'I don't collect these but...', and I don't. But I found this in an antique shop today and something caught my eye.
Now, I don't collect Britain, so correct me if I'm wrong on this one, you GB experts. I had heard that, in the British Post, once a monarch dies and his/her successor is crowned and new stamps are issued, the stamps of the old monarch are demonetized and no longer used. This cover with two King Eddie 1/2 p stamps (SC#230, issue date 1936) was postmarked in 1951, almost exactly a year before the death of his brother King George VI.
Since King Ed abdicated (we all know the story), and was therefore no longer monarch, especially 15 years later, isn't this improper usage of the stamps?
Help me out here...

Note: the cover has an event postmark - British Industrial Fair, 30 April - 11 May 1951, London & Birmingham.
The seller told me the recipient was her husband's father.

Fascinating. I am inclined to think it is philatelic. A nice one!

I though that all British stamps were valid until the introduction of decimal curreny. At least this circulated (philatelic)cover shows that, in July 1969.

As far as I know the use of pre-decimal stamps from one reign to another was not prohibited. I have a 'First Flight' Airmail cover from 1939 with stamps of Three Kings, George V, Edward VIII and George VI, which although doubtless done for philatelic reasons did pass through the normal post, and got to Canada. Today any decimal stamp can be legally used, and certain pre-decimal £ values from the current reign.

How about this one from my "To list on the bay" box..........

Terry

That's terrific. Thanks for the clarification, all.
I had heard this in a talk from one of our senior (most of them are!) and respected members at my stamp club, so had taken it as gospel.
But I had not been able to substantiate it. Now, I'll stop repeating it!

I like the cover anyway...

Queen Victoria stamps invalidated 1st July 1915
King Edward vii stamps invalidated 1st April 1930
All other reigns invalidated 1st march 1972 due to decimalization.