Corporation pop is a euphemistic term from the north of England meaning tap water, so if you want to pay 10,000 times less to slake your thirst than you'd pay for Coke, corporation pop is where it's at. It seems that folk who aren't from northern England find the expression a bit baffling though, so a quick explanation is in order...

First of all, pop is a generic term used to describe any fizzy, sweet drinks (i.e. not sparkling mineral water), presumably due to the popping of the bubbles. If you're American, soda is a close approximation but you'd have some pop or a bottle of pop, rather than a pop.

The corporation part is a little more obscure. In the time before today's huge, privatised utility companies there were local government bodies which handled the water and gas supply. So if you lived near Manchester, your water supply was provided by the Manchester Corporation. And if you're from the North and wondering why you've never heard of it, don't worry: you're probably just middle class ;).


Footnote:
Apparently there's a similar phrase in the US and Australia - Adam's Ale - which means pretty much the same thing.
Sources
UK Food Service - www.ukfoodserviceinfo.com
rec.arts.sf.composition
rec.music.makers.percussion

wertperch says re corporation pop: Adam's Ale is a saying of my grandfather. He was from Nottingham, though he did visit Canada...