Mild is a style traditionally dispensed as Real Ale - from a cask. As a low-gravity session beer, it is intended to be consumed fresh and quickly.
Mild can be successfully packaged in any number of ways: bottle, cask, keg. The low carbonation levels associated with Real Ale do not lend themselves to bottle-conditioning, as it is next to impossible for the amateur to manage the refermentation which provides the condition (carbonation) at Real Ale draught levels. Draught Mild generally has a carbonation level of ~1.8 volumes, and the overwhelming majority of bottled ales have a carbonation level in excess of 2; the difference is notable.
That said, you may bottle Mild successfully, provided you exceed the traditional carbonation levels of the draught beer. In my considered opinion this has a deleterious effect on the beer, but your mileage may vary. Too much carbonation masks the delicate flavors of Mild. But if you use software to calculate your bottle-conditioning sugar addition - and don't exceed 2 volumes - you'll end up with a decent pint.
But next time you're in UK, or a brewpub that does a Mild, do yourself a favor and hit a pub that serves draught Mild. It's an eye-opener!
Cheers!
Bob