A 'jammy bunt' is a rather spiffy way of saying that the person (bunt) in question is rather a fortuitous one (jammy).Jammy bunts are, as you would expect, often in the right place at the right time. Sometimes they deserve it, sometimes they don't. Who cares, they got what they came for!
Luck is involved, for it is at the very foundation of being 'Jammy'. If you don't have a lot of luck, and you wish to become a bunt of jammy nature, getting some would be a good start.
The opposite of a 'jammy bunt', may well be an un-jammy bunt. These unfortunate souls are most likely to spend all their time complaining, and on the whole being very miserable, so if you can, get out there, and try being that jammy bunt that you know yourself capable of being. Put yourself in the best position that you can manage, then let Fate and blind dumb luck be your guide.