The spies were then given a choice of betraying their Nazi leaders or facing the firing squad.
For hanging was the punishment at the time.
A firing squad is, when used, a military punishment, not one for civilian spies even in time of war.
Update: so, someone dares to contradict me in the comments, eh?
Yes, one was shot in the Tower.
All of the others were turned or hanged after civilian trials.
Yah Boo!
Tragic. Because the other thing the Telegraph used to know about, apart from Bongobongo Land, was executions. And World War Two I suppose.
Oh tempora. Which is kind of nice if they use good oil.
Actually they were shot, in the moat at the Tower of London no less. Military trials, military sentences. http://en.wikipedia.org/wiki/Tower_of_London
Josef Jakobs was the last person executed at The Tower of London on 15 August 1941. He was shot by a firing squad composed of soldiers from the Scots Guards.
http://www.stephen-stratford.co.uk/the_tower.htm