Do you know how long it takes to grow a food crop from seed? We're talking months in most cases.
We only have extended seasons for fruit and veg in this country because 1) We buy from all over the world, so conditions in one part might be favourable whilst another is experiencing winter and 2) we have expensive facilities to grow certain crops outside of season, indoors with vast amounts of artificial heat and light.
As others have said, we currently import 60% of our food in this country, and so do lots of other countries (run a deficit).
Then lastly there's the problem of other people stealing your produce and/or land/means of growing your food. Which is nearly 100% likely.
I'm afraid the reality is that a) food doesn't grow quick enough to stop people starving and b) human nature would lead to a blood-bath where people fought and died over radishes and turnip seeds.
Billions would indeed starve in the first couple months. Death typicall occurs after 45 days. After a couple weeks you'll be too weak to work also. Game over.