[AaronLancs]

Given the tweet included a photo of B as well as E, I suspect it might have been a case of mix-up/typo as we know B will be used for the main body of the election night programme. I suppose they could be using it for next day coverage, but seems both unnecessary, as local elections coverage will be done by 16:30 on Friday, and next-day coverage is branded as a Politics Live special so would presumably come from Millbank (if not B).

Only other possibilities would be using it as a secondary studio (though seems over-the-top for local election coverage, especially in a 'lesser' year) or as a fallback studio (though I doubt they would use a studio in the same building for resilience as Millbank a) typically serves that function and b) would make more sense). But, as I say, typo seems likely.

Surely the most likely use of E during the election programme is for news bulletins?

Having a spare studio in the same building can be useful as a contingency against issues which only affect one studio. You don't want to be schlepping over to Millbank every time the automation misbehaves in C.

Random question, how long would it take on average to get from say The World's Newsroom to the BBC's Millbank studios?