As the Guardian says:
His role will be UK-specific, working alongside Mark Howe, who is the UK director of sales focusing on agencies.
Google UK counts "agencies" as a vertical. When you visit you'll find Account Managers who specialise in the likes of Travel or Finance or... Agencies. Mr Brittin is coming in to oversee relationships with advertisers. I suspect this will be larger firms who come to Google directly.
Matt Brittin has come from Trinity Mirror where he was the the Directory of Strategy and Digital. The funny thing about Trinity Mirror is that I always associate them with the icNetwork because of a fairly aggressive sales call I was once ambushed with. Despite explaining again and again that we didn't buy links because we felt it was neither ethical nor in the best long term interest of clients it was one of those calls that would not finish. I need to have two phone manners; one for clients and one for everyone else.
The links there come from the bottom of icWales's homepage. Some links go to partner sites (also within Trinity Mirror) like AdZooks and other links point to competitor sites for Trinity Mirror properties.