What a pity no one has come up with an answer to this. I have searched and searched but without success and I'd REALLY like to know because MY mother used to say, "Right Meredith", in all kinds of situations...who in the world was Meredith?!
Let's keep our fingers crossed, Jane, & hope that someone tells us soon! :0)
I was so pleased to see this question. My Dad used to say this expression nearly every time he got into bed. He used to say it so it sounded to me the same as saying "we're in heaven". I did ask him what it meant, but he didn't really seem to know. It was just something people said during the war when they got inside out of the rain. Googling the expression, I find it was a catch-phrase of a pair of comedians called Naunton (or Naughton) Wayne and Basil Radford. I suspect there may be some connection with the very famous footballer, Billy Meredith.
I've heard that it was part of a sketch that the two comedians used to perform, in which they were the broker's men (bailiffs) trying to gain access to their target's house to seize his goods - every time they thought they'd got in, somebody exclaimed triumphantly "We're IN, Meredith!" - but they always got foiled one way or another.
The phrase originated with music hall comic Fred Kitchen who coined it in a sketch in 1907. It's also the name of his autobiograpy, and it's engraved on his tombstone in South Norwood cemetery. My dad remembers it being in common usage into the 1950s.