I run into this all the time too. The suggestions above are good ones.
The is the Velo Orange long setback:
Another seatpost I've used with Brooks on my more modern setups at times has been the FSA K-Force Light SB32 Di2 Carbon Seatpost. Just throwing it out there just in case.
This is one of the SLK but on the Duell now with a Fizik saddle, you can see it has lot's of setback.
The other solution I normally opt for is to use one of the Gyes type saddles. They have several variations that match the different Brooks models but the key with the Gyes is that have longer saddles to solve the setback problem so many of us run into with a Brooks.
Gyes are harder to find now but they are out there. Of course you already have your saddle so it makes sense to try and find the right seatpost first.
This is a Gyes that mimics the Brooks Swallow. You can easily see the longer rails.
Sorry I don't have any pics I can easily find of a Brooks on a long setback seatpost. Once I discovered how well the Gyes works I went with those and my Brooks sat in the parts bin.
Funny thing is I finally have the opposite problem with the Guv'nor. It's such a relaxed frame that I really need a straight or old school - setback post to get my proper fit in relation to the bottom bracket, LOL!