Where to even begin? Allotments and back gardens can be some of the highest producing forms of food production. Much higher per acre than monocultural farms!
Just pick up any vegetable growing book and you'll get an idea of what is possible. Dowding's
books are a great place to start and offer a sensible and logical way to grow food.
If you're looking to be a bit more adventurous in what you want to grow there are many y great resources. Authors to look out for include Martin Crawford and Patrick Whitefield although there will be many more. There are also numerous blogs and websites with a host of information. One of my favourites is Of Plums and Pignuts which is about an allotment based forest garden in Scotland.
Another option is to visit
local market gardens, CSAs, orchards,
permaculture projects, etc and see what they're up to.