The "allotment" in the UK is like a plot in a community gardens in the US, except with a much longer history.. It's a reflection of the difference in the two nations that a scholar is able to come up with estimates of the total number of allotments over more than a century, up to a million such gardens in a nation of maybe sixty million people. Also in the UK, unlike the US, the national government had legislation on the subject, dating back to 1907, with allotment gardens dating back to the early or mid 19th century.
According to the linked piece, the evolution of allotments in the UK involved differing motivations and rationales: supplying the needs of the working class; serving as a hobby for middle classes; a focal point for socialization; and finally the trendy ecological concerns of recent times.
I show my prejudices by noting this long historical perspective should serve as a caution to US enthusiasts.