If you are willing to/planning to commute into London for work, you will find agencies that specialize in the Marketing field. That would be your best resource for jobs, but don't entirely rule out other or more general agencies. The wider you cast your net, the more likely you are to find a good positon quickly. The West End and Soho are where most of the media/marketing jobs are found. They are both interesting places to work. If memory serves me, I think trains from Kent come into Charing Cross Station, just south of these areas.
I worked as a PA in London, had two degrees, but no previous experience as a secretary. I had a job two weeks after completing a 6 week training course at a secretarial college in Kensington.