Corinne - I joined Roland Berger's London office in the 1990s in very similar circumstances. They were extremely prestigious in mainland Europe - and neck and neck with McKinsey in their native Germany - but far less well known and less established in the UK.
What I would say is that it is really only in the "dinner party stakes" that this has any relevance. You'll get a little tired of explaining to people at social events and college reunions that you work for a firm that competes with McKinsey for strategy assignments, rather than them actually knowing who RB are by reputation. But that's just an ego thing...
... when it comes to getting into business school you'll find they are very well known by the business schools around the world. Equally if you've got recruiters who are looking to hire ex-strategy consultants then they will all know who RB are and how high the calibre of their staff is. So your options whenever you decide to move on are unlikely to be compromised. And the types of projects you'll be working on and the top-tier clients you'll be able to reference having worked with will be pretty much the same as if you'd joined any other firm.
Provided you feel the top people you meet are talented and you're going to learn from them - plus you like the people who they've taken on board there in the US offices already - then I wouldn't have any hesitation about joining them (or indeed any other top firm whose brand in your country is not as prominent as it is in other global markets). My experience is that offices like this are far more entrepreneurial and the scope for rapid career progression/ responsibility is much greater as a result of the "start-up" nature of the operations.
Hope this helps