Learn English – the meaning of “cop” in: “if London cops it, he’ll cop it”

The street in London actually comes from the French "pêle-mêle" meaning:

From various online dictionaries:

1. any old how, at random, chaotically
2. In a jumbled, confused manner; helter-skelter.
3. In frantic disorderly haste; headlong

The French pronunciation is likely where the "a as in gal" sound comes from.

When the portmanteau pall-mall/pell-mell sprouted up, according to Etymonline mall began meaning "shaded walk serving as a promenade". These walkways/alleys had become common places to play a game similar to croquet by the name of pall-mall. The street in question was originally such an alley.

At the same time usage began to drift from the French sounding "a as in gal" to a, perhaps more British, "a as in ball". When we finally got to the point where mall gained it's current meaning, the common usage was the latter but as the street is a proper name predating this change, so I assume it has simply kept the sounds originally ascribed to it.

I've looked up the entry in several slang dictionaries and there seem to be at least two meanings.

• When applied to children, that of silly and poorly educated with a whiff of mischievousness.
• When applied to adults, that of foolish and/or ridiculous.

Below are my sources, cited in extenso.

Oxford Dictionary of Slang (1998)

’erbert n British
A foolish person, a cheeky, unwashed child. For many years, in London working-class slang, Herbert or ’Erbert was used to refer to any otherwise unnamed man or boy. Gradually, probably by being used in phrases such as ‘silly ’erbert’, it came to have the more pejorative sense. There probably never was an eponymous Herbert; it was merely a common working-class name from the Edwardian era.

The Concise New Partridge Dictionary of Slang and Unconventional English (2008)

herbert noun
 1. a mischievous child or youth. Quite often heard as ‘little herbert’ UK, 1999.
 2 a harmless youth; a ridiculous man. An extension of the previous sense UK, 1960.
 3 a man in a specified field of endeavour UK, 1956

John Ayto Oxford Dictionary of Slang (1998)

Herbert (1960) British;
applied to a foolish or ridiculous man ; arbitrary use of the male forename
T. Barling: A dozen baby-brained herberts looking to face me off just to say they squared up to Kosher Kramer before the cobbles came up a bit smartish. (1986)