Hi,
Is this idiomatic expression(idiom?) commonly used in both BE and AE? It's very common in AmeE. I can't speak for BrE.
"lose it" - Is this an idiom?
e.g. - She lost it and threw a tantrum of her own.
The long expression would be 'She lost her
temper' or, longer, 'She lost control of her temper'. It's commonly
abbreviated to 'She lost it'. It's informal speech, and used by younger
more than older speakers, as these things often are.
Best wishes, Clive