That is correct. I suppose that the short term considerations of maintaining enough food to survive and fight trumped all other considerations. The loss of agricultural fertility became a problem after 1945.

Also , I noticed that the health of the poorest segment of the UK population exhibited a general increase in dietary health with the advent of rationing...

On the subject of productivity of agricultural land, see NaturalHomeAndGarden.com article "Pasadena Paradise", where a family in California grow 3 tonnes of food from a 1/10 acre (that's about 40m x 10m), using intensive organic gardening techniques.  That compares with conventional California farms producing 0.2 to 0.5 tonnes per acre in monoculture, such that each American needs 1.2 acres of land to provide their food.  That's 50-100 times more productivity per acre.  The difference is in intensive, manual cultivation, growing crops in several levels from the ground up, in the immediate vicinity of their own dwellings.  

So, IF land is used in this way there is no need for anyone to be short of food, even though the UK climate is a little less conducive than that of California. If people can be persuaded to take up these ideas the country need not have a food supply problem.  At the moment it's too easy to stop by the supermarket, pick up what you need and put the bill on the credit card.  Hence my fear that economic collapse and inability to import food and fuel (as in Cuba) will be an unfortunate pre-requisite for such a widespread change.

For sure. This is often the case, technically there is a solution but the challenge is behavioural and policy. Dr. Mae-Wan Ho from Science In Society has been working on what she calls the "Dream Farm":
"Dream Farm is exactly what we need to feed the world, mitigate climate change and let everyone thrive in good health and wealth in a post-fossil fuel economy"
Details here in this very encouraging article: http://www.i-sis.org.uk/DFHTBCC.php