Two comments. One, 27.0 diameter is often the result of damage to or heat distortion to a far more common 27.2 seat tube. You might have a frame builder verify that you do indeed have a 27.0 or have him ream the tube back to its original 27.2 if that is the case.

And two, setback: Brookes seats are well known to have the flats for the rails well back by modern standards, making pushing the seat far enough back for good fit difficult on modern bikes with steeper seat tubes than used years ago. I don't have an easy, cheap answer for you here, but I paid a frame builder to make me two 60 mm setback seatposts for my two customs (for modern seats but I favor steep seat tubes which allow me to have a rear wheel further forward, improving my weight distribution over the wheels a lot. I end up with the same issue you have, just for a different reason.)

I did find a 25-65 mm setback post that I paid $15 for! ($60 is probably far closer to the norm.) The 25 year old (?) SR MKE-100, a post with a long "rail" the clamp slides on so you can adjust the setback. It comes with a quick release. The thinking was that the rider could push his seat back for crazy mountain bike descents. The idea never took off, but there are still some of those seat posts floating around. Well made posts, not toys. I replaced the quick release on mine with a bolt because I have no need for it and it cleans things up so a tool bag fits. THe post will probably be a much smaller diameter (I think mine and most that I have seen were 26.6) but they shim beautifully to 27.2 with 8 mil aluminum sheet. (Very common. Try any hardware store.)

Ben