Hack away! I want to have as close to an unindictable spiel as possible when talking to people who really know their stuff regarding currency/forex/economics.
Why not just start at the other end?
Why do the USD and GBP have value? Ask your audience to explain that to you... after all, they clearly believe that those currencies *do* have value. They'll probably tie themselves in knots but it's not hard to help them see that a currency doesn't have to be "backed" by something to have value in the real world... so once you've established that things aren't at all clear in the "real" world, you only then have to establish that Bitcoin is at least as good as what we already have.
i.e. you don't need to argue from first principles to win people over.... start from a shared common point of understanding.
To bridge from USD to BTC, I would probably seek to address the legal tender issue first --- use Scotland to demonstrate that it's an irrelevance. Then discuss the adoption/utility argument.... that's just a point in time statement... and you can dismiss most other objections to BTC under this umbrella.
That really just leaves the "why Bitcoin is better" side of the argument, and scarcity is a key angle.... First establish that guaranteed scarcity is a huge difference as compared to currencies with an inflationary central bank. Then demonstrate that the claim for scarcity is credible.
And, critically, the credibility of the claim comes from the incentive structure in place amongst the existing users... if anybody tried to change the 21M coin limit, the incentive is for existing users (miners) to reject this as it would devalue their existing holdings. It is this incentive structure that creates the scarcity.... not technical details of the mining algorith, the cost of power or anything like that.