Is there a travel information office for the USA in London?

I am looking for a USA travel information centre in London to pick up some brochures, especially around Alaska – especially brochures and maps on Talkeetna, Denali and Anchorage and National Parks in Alaska.

That information usually comes from state governments not the federal government. Go to the State of Alaska web site and request info. They will send it to you.
http://www.travelalaska.com/

This form is for international visitors:
https://secure.travelalaska.com/Planner_International.aspx?code=