That's pretty chunky for OX Plat...I don't think it comes in anything with that thick a wall.
OX Plat 1 1/4"/31.8mm tubing comes in with a 0.8mm wall or under. 31.8 ext- 29.6 int would require 1.1mm wall. Even OX headtubes aren't that thick.
Have a look at the downloadable list
from Henry James
for the tube specs.
Is it definitely not an external butted tube, or an internal/external sleeve?
If you're not completely fussed, there's plenty of decent cold drawn 4130 Cro-Mo (TT Verus level) around with 0.035" wall will give you a slightly oversized 29.9mm internal, or 0.049" will give 26.8mm internal which could be reamed out, at a stretch, if we're talking seatposts here.
Pretty hard to find anything in between.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse