In the mid-1930s, while working as a junior accounts clerk at Transport House, he was honorary secretary of the British Workers' Sports Association (motto: 'Be true to your class in sport'). In this capacity he travelled with the British squad of over 100 worker-athletes to the 'People's Olympiad' held in Barcelona in 1936. These games had been organised as a gesture of worker solidarity to counter the rising tide of Fascism and, in particular, the propaganda jamboree of the official Olympic Games at Berlin.
Unfortunately the Fascist tide had risen in Spain more quickly than they knew, and on the night of the British team's arrival, Barcelona erupted in civil war. The team awoke in their hotel to the sound of heavy firing as Fascist insurgents battled in the streets with government forces and other left-wing groups.
The games had to be abandoned, but to mark the Fascist defeat all the athletes marched with their banners to the stadium at Montjuic (the seat of last year's official Olympics) and held an impromptu rally.
With overland travel through Spain impossible, five British battleships were sent to Barcelona to evacuate the athletes and other British nationals. They were all taken to Marseilles but from there had to make their own way back to England. As Underhill recalled, 'I was in charge of the small amount of money that we had. Luckily we had our return tickets so we didn't have to worry about travel, but after we'd paid for our night in Marseilles there was really very little left to see us all safely home.' Underhill's careful budgeting and attention to detail ensured that it was enough.