According to Game (UK), the U-Boat missions add-on is a Game Exclusive, which might explain why it can't be found on any other (UK) websites for pre-order.
Here is a link to it on Game's site, priced at £9.99 delivered:
http://www.game.co.uk/PC/Strategy/~r...AME-Exclusive/
Got my order in already, and Game online are pretty good at getting your order delivered to you on or just before release date.
I hope this is useful.
Cheers,
Kevin.