Looks like the Angels will get them and they'll die of old age in the same kind of closed-loop scenario as that guy in Blink
, with the Doctor unable to intervene and therefore unable to bring himself to see them again. Best of all worlds, in a sense. Also, if they get zapped back into the past to die on the day they leave, then most likely the tables will be turned and Amy will wait for Rory, as she could be expected to live a few years longer than him and so should turn up earlier. Almost a Hachikō situation. Of course, it would be funny if they ended up only a short time in the past and got so uptight about the implied violent death that they end up getting knocked down and killed on the due date. Or not.
I like the Bessie idea too.