My experiences with the Holdsworthy Company (owners of F Grubb, C Butler & Holdsworth names) finished in 1966.
If your frame is listed as 531, I presume it is plain gauge, which would be 20g. This would take 26.8mm. Butted 531 would be 23G at the top of the seat tube. It could be grade A tubing, as they used a lot if it. If you measure the inside diameter beware of distortion caused by brazing. In the time I sold them I never saw one built with the very cheap seam welded tube, as used by Raleigh (Tru-Wel).