what does bimble mean
It's the Oz equivalent of 'going bush'.
Incidentally talking about language, I've seen your forum name many times and always seen it as 'Glen-da-le-goon'. It's not that is it, it's 'Glendale-goon.'
and then exactly what the instructor will say to Southend control to allow us to overfly the threshold, turn 180 degrees, and bimble away the way we came.
Nooo! You mean you're not landing at Sarfend? What sort of xc is that? Where is the bacon butty, what sort of instructor have you got? There's a lovely place called Maypole just a bit further on on the south side of the Thames with a pub right outside the airfield. Ask your instructor person if you can land there for lunch.