Not allowed a shed? How....strange!
We aren't allowed to use hoses for watering, but can happily top up our butts with them. With only 2 taps for the whole site it's one heck of a stretch, but with a bit of luck we will only need to do it a couple of times a year.
For those who are not allowed sheds and can't collect that way, you could rig up a cheap (mine was £6 from Asda I think) plastic Gazebo, and butcher it a bit to have the roof hung upside down, with a drain hole into a butt. I haven't done this but it would seem to me to give at least as much surface area as a shed, if not more. As a bonus, you can fix the gazebo and dig in the shade in the hot summer months!
For best results, you might like to link multiple butts together to get maximum collection in the rainy months. Old bins would work too. I think it's more important to work out how you are going to store your water, than how you are going to collect it - there's plenty of it at the right times of year, but if you are limited to one butt then a week of sun and you could easily run dry.