Did Apple EVER promise these new MacBooks would be available first thing in the morning in every store? I doubt it. If they have some by end of day, they'll have hit their target.
The press release stated :-
"The all-new MacBook will begin shipping Friday, April 10 through the Apple Online Store (www.apple.com), Apple’s retail stores and select Apple Authorized Resellers."
There are none in any UK stores at the moment.