I have three Brooks B17 Imperials on my bikes. All of them are essentially level. When I set them up initially, I simply moved the saddle back if I was sliding forward and forward if I was sliding backwards until I found the sweet spot. The best bike fit info I've found generally says that you should set the saddle height and fore-aft position first based on how they affect the pressure of your hands on the handlebars (i.e. close to zero) and only then do you move on to adjusting handlebars and stem length. I think that folks who have big tilts to their saddle are generally poorly fitted to the bike and need to adjust stem length.

Unfortunately, setting fore-aft position can be difficult with a Brooks because the straight section of the rails is generally shorter than other saddle brands, necessitating experimenting with seat posts that have different setbacks.