The website is a bit ambiguous. the one I read the other week says 28 days from when you tested positive or 28 days from your first symptoms. If you keep taking tests then your last positive test is the last one before you tested negative! I was positive for 7 days (tested negative on a Wed...
I saw the same info but my jab on Tuesday is 27 days after symptoms started (although mine only lasted 6 days and nearly scuppered Marseille). Email came before I cancelled luckily.
I've got my booster on Tuesday but had covid in Sept however the email just received states 'You should also wait if you have a high temperature or feel particularly unwell with any illness. If you have recently got better from COVID-19, there is no need to delay getting vaccinated.' Although...