I don't know about BRexit, but GRexit definitely had a way to make money. Actually, a zero-risk way. The only thing is that you needed to be a Greek to be able to profit.

Betting sites were giving ~1.62x for "no grexit by 1/1/2016"

So, if, say, a greek had 20 btc and placed 5 of them in "no grexit", then in the "no grexit" scenario he would make them 23 in total.

If grexit did happen, then he'd lose the 5 btc of the bet, but the remaining 15 btc would double-triple-quadruple in value in the new local currency after exiting the euro. So it was kind of "profit either way, in different ways" scenario.