.002-.005ish is the spec. Which appears to be the only thing backing pads off the rotor?
So how do you suppose your brakes release after you press the pedal in a stationary car? The elasticity of the piston seal springing back into its normal shape is what backs the pistons off the back of the pads on modern brakes. The original Dunlops has retractor springs because the old piston and seal config didn’t retract so well.
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Well yes, they release enough (from stationery) to allow motion, because the master cyl is no longer applying pressure. But the pads are still dragging, as you move off. Just as when you remove the wheel, you can easily feel the drag of the pads on the rotor when it’s rotated. Nothing’s really pushing them off the pad enough to keep them from binding, that I can see, save rotor warpage and /or bearing run out.
Have a good read on disc brake technology. I can’t type it all out.
The stationery/stationary issue is pretty straightforward too - on paper 
The solution is called SQUARE CUT SEAL.
Which XJ do you have? I have a Series 1 with refurbished brakes. It rolls with ease and brakes never bind, much better than my Mk2.
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Unless the pistons themselves are binding, preventing proper retraction by the seals, the drag itself is immaterial to pad wear and heat generation.
Considerable pad pressure is required for any effect, and without master cylinder pressure there is none. Also, the disc runout and wheel wobble (hub endplay) will usually help to kick the pads off the discs. Admittedly only with the car moving - but that initial drag is brief.
As an aside; the retraction should indeed be minimal. As the initial pedal movement is used to bring the pads in contact with the discs - and excessive retraction increases initial pedal movement. Certainly; excessive drag may imply something is wrong…
Frank
xj6 85 Sov Europe (UK/NZ)
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XJ6 SIII
Regards,
Ted
Flex in the spindles could be added to the list, although probably not intended by design. And less so in the later cars with heavier spindles.