Well I can only speculate, but yes, it will hopefully be made available on the Apple store straight after the Keynote, just like the retina MBP, latest MBP's, latest Airs, latest iPad and most other launches that I can remember. Though we don't know if the 27" will be available straight away, I hope it is.
It's so much fun, people are tearing their hair out because the store is terribly slow when they put it back up.
As for buying in store, it depends on demand and supply, with the rMBP, none of the stores had any for about 4 weeks after launch. My local store is Birmingham too, at just 40 miles away.