I believe that the Hetchins production records show that almost 500 frames were built during WWII. They would already have had a stock of tubes, and no doubt Hyman and Jack used their contacts to acquire more. Accles and Pollock had advertisements in the April 1941 edition of Cycling headed "Cancelled for the Duration", and stating that as their tubing was so suitable for aircraft production there would be none available to the cycle trade until further notice, while Reynolds also advertised saying that "all their production had literally gone up in the air". On the same page Renold and Coventry chains had an advertisement advising how riders could make their chains last longer to help the war effort. Elsewhere there are advertisements for new cycles from Hetchins, Buckley Bros, Raleigh, BSA, Rudge, Gazelle, Higgins, and Baines. The editorial of the same issue reminds cyclists that although there was a requirement in force that motor vehicles were to be disabled when not in use (usually by removing the distributor rotor arm), the same did not apply to cycles. However, in the event of an invasion cyclist in areas near to the point of invasion may be required to hide or disable their machines.
A friend works for a company which restores vintage aircraft and he tells me that Reynolds tubes in various plain gauges were used from before WWI, and at one time they had stocks of 531 in long lengths which they sometimes supplied to a local builder (Northamptonshire, but I don't know the name of the builder) if he needed to build a large frame for which butted tubes were not available. They now use 4130N tubing as a replacement which is made in Imperial sizes and various gauges.
Peter Brown, Lincolnshire, England