ALB wrote: “Don’t expect him to say anything much socialist as he was one of the few party members who joined up to fight in that imperialist war. As he joined the RAF he was a volunteer not a conscript.”
I was always under the impression that he joined the SPGB after the war, so it wasn’t a case of him resigning from the SPGB to enlist.
However, it should be stated that he was already aware of the SPGB as a kid, so it’s not like he didn’t know the Party’s position on war before enlisting.
For more background on Barltrop first coming into contact with the SPGB, see this excerpt from The Monument: