I know there's a lot of discussion about Australia, but one major difference is that they're the big regional power in that area. Most boats will be coming from much poorer countries, who have little power to challenge Australia.
Any similar policy in the UK, would involve sending boats back to France/the EU. France is one of our biggest trading partners, while 40% of our total exports go to the EU. When you consider all the other partnerships we have with France and the EU, it's a huge risk.
As you say, Australia isn't considered a rogue state, but sadly I suspect that self-interest tends to trump morals for many countries. If the UK were to try and start attempting* to return people to France, it wouldn't be just looked down upon morally, or result in a few frowns on the continent. We'd be burning major bridges, and the consequences could very quickly outweigh any savings costs.
*and I say attempt, as I don't see how it works in practice if France refuses.