Royal Mail Gearing UP
More than 500,000 advance order copies of “Harry Potter and the Half-Blood Prince” were delivered July 16, 2005, in an operation that required 150 additional trucks ferrying books to 1,400 delivery offices across Britain.
And while there is no set date as of yet for the seventh book, the Royal Mail is trying to anticipate and get ready for the last of the series.

“We’re already planning for the launch,” Royal Mail spokesman James Eadie said Thursday. “We’re talking to retailers about what volumes they expect, and what they expect from us, so that we can put the logistics in place to deliver on the date of issue.”
The movement of the trucks is being coordinated in advance to avoid congestion, and local post offices are also considering bringing in extra staff to deal with the overflow of books, Eadie said.
About one in every 50 British households ordered a copy of the book last year, with one in every 18 addresses receiving a copy of the book in some neighborhoods.

So, it seems safe to say that everyone who orders in the UK online, that their book should definitely get there when they say.
Ordering the books online seems to be a growing trend. Especially when one thinks about the huge lines and annoying, pushy people you'd have to be dealing with. In fact, after reading this article, I think that's what I'm going to do as well now. Who cares if I'm not the first to finish the book, as long as it gets here the same day.:)
**This from Journal Gazette