Hello, I am an environmental engineer sharing my design for a ~300 gallon IBC tote rainwater collection system. What makes this different and beneficial for self-sufficiency is the low cost per gallon. This tote also provides a large enough storage tank to use a dedicated irrigation pump for reuse. You could make this as simple or as complex as you wanted to.
IBC totes are frequently used for storing and transporting food grade materials, so there is an aftermarket for cheap used totes, you just have to verify what they were used for. I got this 275 gallon tote off of marketplace for $100 USD. It was previously used to transport food dye. They are highly durable because they’re built out of HDPE and galvanized steel.
Other Materials:
Leaf filter, from $10 to $70 - my preference is the Leaf Eater Advanced ($66)
Downspout diverter, my preference is Earth minded Flexifit diverter ($20)
PVC pipe and valves, figure $50
Total entry cost including tank: $180 - $240
Optional:
Lid adapter for two inlet ports $10
1 HP irrigation pump $140
Discharge splitter $10
Wi-Fi controller $30
Piggy-back Level float, alternative controller $50
Ozone injector to keep the water fresh and odor free: $400 (luxury) JED Engineering model 203
Tbh even my fairly small garden could make use of this much water in the UK. I focused on keep stuff alive rather than optimal growing conditions.
Problem I find is that collecting rainwater is difficult to justify on the price of it. I looked at tanks out of curiosity, but water is so cheap that it could never realistically pay for its self.
Space is also difficult as my garden is fairly small. Looked at underground tanks but the prices there are even more crazy. I think the most cost effective option for me would be to just dump greywater on the garden instead. That just costs a pipe. Maybe something to put over the pipe where it crosses the path to avoid a trip hazard.