Hi there. To be a bit more helpful, search Google for their names and go to their official websites. Don’t use a general job or recruitment website as they usually work on commission or even insist on an upfront fee!
I’m assuming from the brand names, that you’re looking for somewhere in London, as most of the Head Quarters for those companies are there. For instance, the page for Ogilvy is at www.ogilvy.co.uk/index.php/vacancies/
It seems like a speculative role as it just asks to send them your CV. Another good brand to look at is www.360inspire.com they are a recruitment advertising agency based in London.