Raising it up on stilts is the way to go if your allotment gets flooded.
1. Ask experienced allotment holders at your location, how deep is the worst flooding they have seen? If the worst version of the story is water 2 feet deep, plan for 4 feet deep.
2. How much shed do you need? If you are only planning on room for some hand tools, fertilizer and nursery pots, you need much less, there are sturdy lockable sheds made of plastic resin available for $150 and up (guessing that is roughly £100 and up). if you are keeping your own lawn mower and rotavator on site, you might need larger. There are metal sheds as well as resin sheds, buying one may cost less in the long run than building it yourself if you don't already have tools. Materials also cost a lot right now, under COVID. Estimate the size of shed needed, plan for a little bigger if possible.
3. CAVEAT: I don't know your local building codes, determine any local requirements before designing and building any structures, modify my instructions as needed to meet code. To create a platform on "stilts", determine the height from step 1 (above), horizontal "footprint" dimensions from step 2. All wood materials used for your platform should be exterior grade, pressure treated. Carefully measure where the corner posts need to be, dig narrow holes roughly 0.5 meter deep for each post, construct your platform with the posts in those holes. Dry fill concrete mix in the hole after the platform is built and is level. Pour water into the concrete, use a wood pole to push up and down through the concrete to mix in the water. Depending on height, you may also need to build steps or a ramp into your shed. The concrete provides a firm footing, also anchors what you have built during a flood.
4. If all this sounds like too much trouble and cost, consider just bringing tools from home in the boot of your car.