Wander round any local industrial estates and see if anyone has any ibcs or big blue barrels outside- its really hard to get free stuff off of companies in the UK, given waste-processing licenses and all, but its worth trying!
Also ask any local community gardens or allotments- they tend to know the people who might be giving away rain barrels and things. I find the big blue barrels are easiest to get hold of.
I'd also ask on gumtree or freecycle- there's quite often compost bins and water butts going on ours (I'm in the Midlands though).
I got mine from the local 'wilkos'- all purchased a broken or leaking waterbutts for about £3 for 210 litres, fixed them up with glue! It helped that I used to work next door though- so I could keep an eye out on the reduced trolley, and get to know the staff so they would save things for me.
That sounds cheap compared to here. They are running $80 each for food grade ones in the midwest US.
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