Note: Allotments are not limited to the UK. In the Netherlands, where high density living in apartments is more prevalent than the UK, Allotments are very popular. The Dutch call them Tuin, which translates literally as "Garden". They tend to be stricter and more regimented than UK allotments, and also a lot more expensive, we used to pay several hundred euro. The Dutch also tend to use them slightly differently. While many will use their Tuin to grow vegetables, flowers are more likely. They really are just gardens, more than the food production garden that they are in the UK.
Finally, a bit of allotment trivia. Einstein used to have an allotment/tuin when he was living in the Netherlands, and he got a letter from the Allotment society asking him to either tidy his tuin, or to vacate the plot.
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Julia