I have taken a quick walk through the UD when I was flying BA F, and this was during the flight. I had never been upstairs on a 747 and was curious, and might not get a chance to ever fly in one.
When I did it, I asked the FA at the bottom of the stairs if it would be okay to take a quick look. She could see I came from the F cabin, if it mattered. She did walk up the stairs ahead of me to let the upstairs FA know. I followed her up the stairs and took a quick walk up towards the front.
It is probably a YMMV situation, but being in F and asking politely will likely go a long way. Given the cockpit door is up there I knew it wasn't a sure thing, so I made an effort to phrase my request as a question/favor.