From Charles Byrne, the Royal British Legion’s director of fundraising: “We wanted to end the debate about how to wear a poppy during this year’s Poppy Appeal. The way you choose to wear your poppy is a matter of personal choice. As long as you wear it with pride, then you’re wearing it the right way.” They even released a video this year, doing a tongue-in-cheek debunking of all those myths.
Personally, I think they look nicest with the leaf pointing the other way, in line with the jacket lapel. I have no major view on whether the petals are sideways or vertical, though.