re "I can't seem to figure out the pun ... is it a British-ism"?

Yes, it is a British-ism.

There is an old North of England saying:

Where there's muck there's brass

Don't know what "brass" means, but the saying must be well known if a twist on it became the Guardian's most admired pun in 2003.

From this Guardian story, introduced by the headline "Where there's muck there's brass", I take it that "brass" is "money" or "cash".

http://www.guardian.co.uk/climatechange/story/0,12374,1232057,00.html

Yep, that's it: "money".

This from:

A dictionary of slang - "B" - Slang and colloquialisms of the UK.
...brass Noun. 1. Money. 2. Prostitute.

The "prostitute" undertone makes the "bras" substitution all the more clever.