Blarst garl!
Yah carm frarm Saarfik, an yew darnt think thart yewv gart an arksent?
Or, if you'd prefer a serious answer:
I understand 'posh' people because:
(a) that was the type of pronunciation that my schools tried to teach me (just down the road from you, in Ipswich); and
(b) such people frequently appear in films, on radio or on TV.
I can also easily understand most British accents, partly through studying for my degree alongside people from all over the country. (I'm sure that, when you get to University, you'll have a similar experience, Molly). However I have to admit to occasionally struggling with broad Geordie and Belfast accents, along with some of the broader Scots accents.
Chris