But even if dates are published no one is going to start re revising for an exam until they've got their results. So needs to be a good 3 weeks between results and resists, even if dates are known ??
This isn't really what resits are supposed to be for, though. The point of resits is to allow a first attempt for people who were ill or otherwise incapacitated on the day of the exam, and a second attempt for people who for whatever reason underperformed on the day. They aren't supposed to be a second attempt to learn the stuff you were supposed to have learned before the original exam. The gap between first sits and resits should really be as short as possible, though obviously not so short that someone who was acutely ill on the day of the exam is unlikely to have recovered.
As this case relates to a medicine course it is possible that either: (a) they operate a system of programmatic assessment, which wouldn't normally include resit attempts, but have put in a resit at short notice as part of the university's safety net because of Covid disruptions; (b) they had always planned for a short gap between first sit and resit.
In fact, I've just looked it up and Bristol uses progress testing, so it's likely that students have sat tests throughout the year (usually it's four times per year; I don't know whether this is the case at Bristol). Usually, a small number of students will be judged to to have demonstrated insufficient progress by the end of an academic year and may be required either to leave or to enter extenuating circumstances to be allowed to repeat the year. Again, university safety nets will have led to resit attempts' being introduced at allow a possibility of rescue, but there's no reason why a long preparation period should be offered. A fundamental point of progress tests is that you're not supposed to revise for them.