(Usually) council owned land divided into small patches that people can rent to grow vegetables. Often dotted with sheds and such. Not a terrible place to hide in theory. Though saying that, they're often populated by nosy old biddies who'd clock an interloper immediately and be absolutely delighted to phone up the Met and shop him in.
They become very popular during WWII and are commonly one of the few areas semi-private but accessible unbuilt-on land you find around really urbanised estates and cities. Very very hard to get your hands on a plot these days or so I'm told.
View attachment 480646
Typically look something like this.