If only! All my purchases of Canon's big whites have been based on careful perusal of threads like this. Only when I visit WWT sites such as Slimbridge and Welney have I seen big whites - and both of these are a fair drive from home. The local shops certainly don't stock them, and whilst Calumet may have their HQ in Milton Keynes, I get the impression that their stock is for sale - not demo. It's one reason why Birdfair and Focus-on-Imaging were so important.
However, last Sunday at my local reserve, a guy turned up with an Eos 1DX mounted on an EF 600 F4L, latest version, I expect.