Green’s Dictionary of Slang

(UK black) a general excl., used for anger, surprise, amazement, approval, envy, etc.

2003		Dizzee Rascal in Vice Mag. at Hyperdub.com 🌐 England feels crammed… there’s no space… like, rah, I want to get out, it’s fucked [...] We express what people can’t express, when they ain’t got no way, so they feel like ‘rah, someone relates’.
2017		hubpages.com ‘Roadman Slang 4 Jun. 🌐 Rah - this is the most common sound of approval in London, and is pretty much void of any tangible meaning. [...] It could be translated to ‘nice’, ‘cool’, ‘sick’ etc. It can also be used to indicate emotion/frustration/excitement. e.g. ‘rah, that test was hard!’ or ‘rah, my mum's leaving tomorrow so the house is free!’.