It all boils down to your allotment rules, and they can be very different from site to site.
Some are like having your own personal garden, often called Leisure gardens & allotments. These types can be just flowers if that's what you want to grow.
Others were set up, mainly, so people could grow their own vegetables and a limit is set on the area allowed for flowers.
Here in Ashford we are only allowed 25% flowers and lawns are not allowed for a seating area, or anything like that.