Interesting - I had no idea there were two different editions. The edition I bought was the UK edition which I ordered via Book Depository, a UK company that sends out its books worldwide with free shipping. It's my go-to for any Doctor Who books. Shada isn't scheduled for retail release in North America until the summer. I've never had any issues with customs, etc. but I'm in Canada so whether that would apply to someone ordering from the US I can't say.
Presumably the North American version will simply have smaller type. I can't imagine it would be edited at all. But as someone who has had to shrink and manuscript by 100 pages, and having looked at the size of the type in the copy I just got, if they're going to reformat it to trim that many pages, I hope the book comes equipped with a magnifying glass.
Of course the 336 number could simply be a typo too. For months Amazon and other sources listed a Doctor Who novel titled Apollo 13 being published when in fact it ended up being titled Apollo 23. And Amazon still lists an Enya DVD release for sale that was cancelled 12 years ago. So just because Amazon says its 336 pages doesn't mean it's actually going to be 336 pages.
Ace Books? Since when? BBC Books has always published North American editions of its books, whether it's the Target reprint, the NSAs or things like Coming of the Terraphiles and Silent Stars Go By. Is this new? Amazon shows a company called Berkeley so has BBC Books lost the rights? Cover art is the same.