And these laws are FUCKING RIDICULOUS.

Isn't that normally for water that has landed on the ground rather than on your own roof? What if you just dug some trenches to collect the rainwater in the land it just happens to collect in a pond

You can also buy IBCs with galvanised sides, which is pretty cool. They can be £10-20 more expensive, but makes the whole thing more robust, in my opinion.

Eg: https://smithsofthedean.co.uk/product/ibc-tank-1000-litre-galvanized/

Ours have little holes drilled in the front side every 100l to expose the plastic tank underneath, so you can see the liquid level.

I would recommend this type so you don't need a separate cover, which could get ripped etc.

No it is rigid. Just search for "IBC container" and you will see how it looks under that cover. Those tanks are widely used to transport liquids.

Our UK garden is about 75x75m, which is a decent size, but not massive. About half to 3/4 of it is grass, which I hardly ever water, and haven't this year even though we had a drought.

We have 5 IBCs, and several other randomly-sized water butts (~7000l total (1500gal UK, 1850gal US)), to keep our veggies and fruit bushes watered. We ran out this summer, and had to use about 500l of tap water until it rained again :(

Dry periods are likely to get worse in future, so I need to get more water storage. I also want to start using rain water to wash clothes (or at least fabrics we use for our pet rabbit houses), but I need a good filtering system first.

We have 2 IBCs stacked, and I was trying to use the top one to gravity-feed our downstairs toilet cistern for flushing and worked well, although it took 7 minutes for the cistern to fill, which isn't great if you have to accommodate a lot of people. It is good enough for us though.

I could add a pump to speed up the process, but I'm trying not to add more cost and complications to the system unnecessarily.

I can fit 1 more IBC in somewhere - they don't look pretty, but they are very practical. I normally transport them home on the top of my car on a roof-rack.... and drive slowly! :)

When there isn't much rain in the summer and you try to keep your plants alive with collected rainwater, I assure you that it empties fast

It's just water bro. Shower, laundry, drink it, whatever. I have a thousand gallon tank, nice family home might have 10k. No other water source

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.

I am not the OP, but even if it doesn't have practical use, I just think this is coolest thing ever. It also aligns with self-sufficiency and resilience values of this solarpunk server