Hi Jack,
I know there are other people on here who don't have water laid on at their allotments. If you have suitable transport (and your tenancy allows) you can try to get hold of an old shed through freegle or free-ads. People very often offer if someone is willing to deconstruct the shed themselves and remove it.
Alternatively, if you are handy, pallets are a wonderful thing which you can usually get free of charge from builders merchants, freegle or go for a drive around your local trading estate and ask. You can build a shed, toolchest and/or compost heaps easily. Look at the pallet thread on here (
http://chat.allotment-garden.org/index.php?topic=63335.0) Although we built compost heaps and "fencing" with pallets, we ended up buying a new shed for our plot as there didn't seem anything free or cheap on offer anywhere when we were looking.
Once you have a toolshed or toolchest in place, you can run a water butt off this to collect rainwater. Water butts could be bought, old food barrels, bowsers or old water tanks. Although we have water at our site, we tend to use our water butt water first if it is available (other than for seedlings in our greenhouse which we always use tap water to water).
Another idea of collecting water, if you have room and no children, is to dig a hole in the ground (big enough to dip your watering can into and drape a large tarpaulin or heavy plastic sheet over the hole, working the sheet into the hole as a catch-bucket. It is crude, but it will work.
Once plants are established, unless there is a complete drought, plants do better without being watered too much as it can produce soft sappy growth which can be irresistable to pests such as slugs. You can also use mulches, bean trenches, lots of manure dug in and other strategies to retain water where possible.
I guess it depends on the rental charge of the plot - some sites have fabulous facilities, but charge a lot more than sites with little facilities laid on for plotholders.