I'd say her family's accent is a mix of Southern British Standard (common in the South of England generally) and Hertfordshire (which may be close to where they live, not sure of their location). Ruby's accent is a lot closer to Upper Received Pronunciation (what people would call the Queen's English or a 'posh' accent). That's why it's so jarring when she mispronounces words, but you don't expect it from someone with her accent/sound system.
Source: If you can't tell, I did Linguistics as a degree