Is it an unusual British slang?

I think I know what it means, a new pee, or more correctly "p", was what the penny became called jokingly for a short while when the currency changed to decimal in 1971, to distinguish it from the old penny. There was an older expression that if something was "two a penny" (a shortened form of "two for a penny") it was very common, often found, nothing unusual, cheap, you could buy two of them for a penny. So it looks as if the sentence is just saying there were lots and lots of these "long haired wayward wasters twenty years ago", i.e. "two a penny, or two a new pee", pee of course also being a pun on taking a pee, it was never the official spelling of the new penny.