As of yesterday, the Downtube Mini is available to purchase from Velo Chocolate. It's priced at £350 including shipping. We've put it that way because, as it's in America, if we gave the price ex-shipping, it would seem very low, and then the shipping cost would come as a nasty surprise. The shipping is, in real terms, about £100, but even with the shipping included it beats its main rivals on price. Bromptons seem to start at about £500 for the 3 speed, and the Dahon Curve is £350 for the 3 speed version before shipping. Factor in the 8 speed Sturmey Archer hub, and the Mini looks like the best deal on the block!
Given that the bike is shipping from the States, you should be prepared for a potential tax bill following it into the country. This sometimes doesn't happen, for reasons we can't fathom, but often it does. This will be for VAT against the purchase price ex-shipping, and given the amount of the actual price you pay is made up of shipping and assembly and tune-up in the US, it's not too high.