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.