Possible to install Outdoor Tap to rear of mid-terrace property?

You can have a tap there as long as you have a water feed which can be run to the rear of the house . So the main to the water tank is an option

Yes no problems to fit at rear of house jus make sure to use a tap with a non return valve inside or fit 1 to pipework.

you can fit an outside tap more or less anywhere as long as you can pipe it up to a water feed. However bare in mind exposed pipework should be lagged to stop it freezing in winter (it should be isolated in winter when not in use). I would try and feed it from the mains rather then the tank personally. it must also have a double check valve either built into the tap itself or on the pipework as per water regs to prevent back flow