Big companies to work for in London

Hey, I'm thinking of moving to London in September through the HSMP process. I have experience in the Finance area, and am looking to get a job with one of the bigger companies in London.

Just wondering if anyone works/has worked for the larger companies in London. I'm trying to find a company with an overall younger work force (20's-30's). I've been looking at positions available at O2, Vodafone, British Airways, Marriott. Also the big advisory firms like PricewaterhouseCoopers and KPMG.