Yes and No: back when this frame was (probably) made Reynolds supplied Plain Gauge 531 tubesets in 3 different gauge wall-thickness: 22, 21 and 20 swg, these all in Imperial dimensions so a seat tube of 1.125" OD. If you have a frame with the (shifting to metric-speak now) wall-thickness of 0.7mm then the seat tube opening would be 27.2 and typically reamed for .2mm clearance so a 27.0 seatpost. This is the same wall thickness at top of seat tube that later became used in the 531ST (Super Tourist) tubeset, and those took a 27.0 post, but the OP's frame is a little early for ST tubing, I think. The other 2 plain gauge walls would result in typical seatposts of 26.8 or 26.6 (for the medium and the heaviest gauge, respectively), but I don't think 26.4 would be 'correct' unless the heaviest tube got no reaming (or was in fact a 531Metric tube).
Of course, as said, OP should check and confirm that a 27.0 is the right size for this frame, my gut would say it's a nice enough frame to have been built with 531DB and so should take a 27.2, in an ideal world.