Our allotment is on privately owned land ; the land owner specifies no permanent concrete installed. But loose slabs (under a shed) are perfectly removable therefore permitted.
So THAT is the issue you need to pursue with the allotment committee/or the owner. Be polite, co-operative, and do it in writing.
Allotments function solely on the application of communal knowledge, understanding, communication, co-operation, skills, muscle and mutual support. You need to adopt that allotment mindset; of self help, and communal can-do.
Forget hiring commercial companies/ paying a little man/ buying stuff off the shelf. That doesn't happen in Allotments.
You're now in the world of Freecycle, improvisation, adaptation, making-do, taking risks, gambling on weather, tears, disasters, surprises, lots of fun and satisfaction.