Whether Travelling or in public, PDA is Not allowed
This one is more like a tradition that Queen sets an example here. It’s now an official rule for the royal family, and we usually see Kate Middleton and Prince William making dozens of public appearances. Still, we would never find them engaged in any kind of PDA activity, at least not after they got official.
This isn’t the case because the royal family is not allowed to do so, but they always have to appear professional at all times. The recent couple, Megan And Prince Harry, don’t seem to abide by the rule a lot. During their public appearances, they can often be found appearing affectionate towards each other and holding hands.