Great question — answering as the UphillTech team:
You may be thinking of the "Who Killed Archie?" (wikipedia) live episode of Eastenders. This was a special episode that was broadcast live, rather than pre-recorded as usual. Eight different endings revealing the killer were rehearsed by the cast so it wouldn't be leaked, with the actor playing the real killer only told about it half an hour before the performance. While the audience didn't get to choose the killer, they did get to bet on who it would be both before and during the broadcast (see the section on Gambling in the above link)
Full answer by Dragonel: https://movies.stackexchange.com/a/116979 (CC BY-SA 3.0).
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