27.2 seatpost is correct. All Bates were built with 21/24 gauge butted tubing. Only way Reynolds made the Cantiflex tubes. In metric that is 0.81/0.56. With an OD for the seat tube of 1-1/8 or 28.6. Do the arithmetic. Heat distortion takes 0.2, any frame.
It is extremely common for seat lugs to get mashed somewhere in a long history. About 999 times out of 1000 it is easy to bend them back to where they belong. This one is a little bit of a surprise as the Bates seatlug is massive. But only a little. Since this is such a special frame, find a frame builder or old school steel bike mechanic who can show you the ropes. You could ride it a little while with a 27.0, some risk if done long term.
One way to damage any steel frame is to ride it without a vent. Domed top to seatpost is pretty, and then condensation stays inside. Even on a stored display piece tubes will rust from inside. Either an open top seatpost or drill a vent in bottom of BB shell. On a rider it is entirely kosher to drill the most antique shell. Or the most antique seatpost.