The backflow is to protect the mains from syphonic action should a mains burst and negative pressure result in the mains sucking water from the hose, the tap may be connected to, back into the mains.
The backflow protection must be protected from frost. Being outdoors means it is exposed to frost, so this is no good. You will still NEED a double check valve indoors with this tap.
How the hell do you get water to "back flow" against mains water pressure. .??
I'd be installing a double check valve indoors anyway as per WRAS guidelines
Walk into a plumbers wholesaler and buy much cheaper than this. Don't be scared of wholesalers they won't bite and will be really helpful.