It's not impossible to get inside the advertising world in london, it's just difficult, as the place is incredibly competitive.
Get a job in advertising in London. Do you really realize what you want to do? Be aware that many people from all over the world just crave for that: Bresilians, northern americans (miami school, ...), french, spanish, italians, eastern europe (poland, russia, ...), koreans, japanese, chinese, and all the (former) commonwealth countries: indians, australians, new-zealanders, south africans...
Not to forget all the english folks (Watford, St Martin, Bucks & many more) pressing in there ;) Very multiethnic blending as in london in general, as the only entry ticket is the language, which is common.
The deal in London, what's it's all about, is big ideas. Forget your book and get ready for a new one: Paper and Pencil strategy!
If you like the sound of names as Mother, BBH, CHI, DDB, TBWA... They want simple, strong idea which you can decline on any media, from below and above the line.
