That is, sadly, what this thread is all about. If you can't get your saddle far back enough, it's because your seat tube angle is too steep. But "modern" frames typically have steeper frame angles than were common when the Brooks saddle frame was designed (the standard B.17 frame has not changed in 75+ years). Therefore "modern" saddles (Selle Anatomica, etc) have longer rail that allow the rider to compensate for the "modern"ness of their frames. Unfortunately, the "modern" saddle rails, being longer, make the nose of the saddle wider; causing more thigh friction and other discomfort issues.

Another solution is a seat post with extra setback.

Another solution is a different diagnosis: it's not that your saddle is too far forward, but that your handlebar is too far back. Get a longer stem, shift more weight to the front wheel, eliminate your setback issues, and maybe even improve the ride of your bike.