the beginning of the end.

[Tartar Horde]

NR brings up an important point when he talks about "wheat" and not understanding it's growing requirements, this is also very pertinent to other foods we take for granted. If we are looking at a total breakdown scenario then I believe we in the UK are looking at a serious famine, here's my reasons why.
In our society very few people grow our food, and that leaves a small knowledge base that will disappear. Very few people know how to keep and propagate their own food, either from seeds or cuttings, and allied to this is the knowledge when to plant certain crops. I see a country where our variety of crops and vegetables reduce in number due to the lack of knowledge in the general population, and only surviving with any variety and abundance in those people/families/communities that have the resources and knowledge to grow them.
An example where knowledge could save your life, and ties in nicely with NR's "wheat" comment.
A prepper family stumble across a sack of Wheat and plan to plant it next spring so they can have an Autumn harvest, not realising that modern varieties require "vernalisation" and so are planted in late Summer to overwinter and grow again in the Spring. The family thought that just like days of olde you plant wheat in the Spring and so their harvest failed, or was greatly reduced. I see the same problem with food like Potato's with the gradual diminishing of the varieties and number.