What did Britain do to WWII spies?

In "Allied," Brad Pitt portrays a Canadian WWII officer in London whose wife (Marion Cotillard) is suspected of spying for the Germans. His superiors set a trap for her and warn him that if she fails the test, he will have to kill her himself or both will be executed. Question: Could this ever have happened? It's hard to imagine the UK sanctioning casual murder, and in the particular circumstances described, it would have made more practical sense to try to feed an agent false information or try to enlist her as a double-agent. Pure fantasy, or factual foundation?
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