Brooks saddle angle dilemma
I noticed the same thing when I first got a B17. I fiddled with various angles and so forth, but in the end I found it's mainly just a matter of getting used to the smoother feel of leather as compared to the plastic you're used to. You slide over leather more easily, especially if you wear typical bike shorts, and this gives you the sensation that you're going to slide forward since you're not used to a smooth surface. But you won't. Your position in the saddle is maintained by your arms and core no matter what sort of saddle you have, not the saddle itself. The smoothness of a leather saddle is actually a benefit, not a drawback, and you'll come to appreciate it if you give it time.

Yes, breaking in helps a bit but it's not necessary. Just getting used to the new feel is all that's necessary.

After a lot of trial and error, I ended up setting my saddle perfectly level just as I had with my plastic saddles. The temptation to tilt the nose upward just because the material is smooth should be resisted, as you've already discovered.