So, final answer: No, your lipstick is not a liquid when flying in the UK. You can keep it in your handbag, your jacket pocket, or your lucky travel sock. You do not need to squeeze it into a clear plastic bag next to your hand sanitizer.
But here’s my pro tip: if you feel nervous, just pull it out and put it in the tray anyway. It makes you look like a super-prepared traveler. Or, you know, just put it in a baggy to avoid the anxiety. Sometimes it’s not about the rules. It’s about winning the mental game with airport security.
Go forth and fly, glossy-lipped and rule-bending. Just don’t bring your full-body moisturizer in a jar unless you want to have a very interesting conversation with a man in a high-vis vest. And if you do get stopped? Just smile. You have excellent lipstick for it.