Awful innit! (sadly true as well really, I think 25 years is more than long enough to wait for free tax especially considering the state of the roads).
While the makers of Ad Blue are definitely taking the p155, charging £10 for 5l (IIRC). The main ingredient is urea! If I could get a suitable filter (and I had a vehicle that needed Ad Blue) I would urinate through the filter into the specific tank. It would be a lot cheaper, and probably work just as well!
I've heard of Americans trying to use moose pee as an alternative, although I think I'd rather pay for the real thing than go to the trouble of trying to work out which hose fitting to use to "harvest" a moose!